Computation of Earnings Per Common Share and Common Share Equivalent - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Basic earnings per common share & common share equivalents outstanding:
Net income (loss) attributable to common shareholders	$ (1,043,689)	$ 70,601	$ (589,226)	$ 136,268
Weighted average common shares outstanding - basic (in thousands of shares)	257,022	273,660	261,203	282,442
Basic earnings per common share & common share equivalents outstanding	$ (4.06)	$ 0.26	$ (2.26)	$ 0.48
Diluted earnings per common share & common share equivalents outstanding:
Weighted average common shares outstanding - basic (in thousands of shares)	257,022	273,660	261,203	282,442
Impact of share-based compensation	4,276	3,434	4,063	3,683
Weighted average common shares outstanding - diluted (in thousands of shares)	261,298	277,094	265,266	286,125
Diluted earnings per common share & common share equivalents outstanding	$ (4.06)	$ 0.25	$ (2.26)	$ 0.48
Dividends per ordinary share	$ 0.22	$ 0.20	$ 0.66	$ 0.60

Aggregate dividends paid during the period for each share of common stock outstanding.

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation.

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