Annual report pursuant to Section 13 and 15(d)

Debt (Exchangeable Senior Debentures) (Details)

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Debt (Exchangeable Senior Debentures) (Details) - Charter - Exchangeable Senior Debentures - 1.75% Exchangeable Debentures
Jun. 18, 2018
$ / shares
shares
Debt Instrument [Line Items]  
Interest rate 1.75%
Exchangeable ratio 2.6989
Exchangeable price (in dollars per share) | $ / shares $ 370.52
Number of shares exchangeable (in shares) | shares 1,288,051
Redemption price (as a percent) 100.00%