Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share [Abstract]
Net income attributable to common shareholders	$ 53,246	$ 263,027	$ 121,308	$ 4,453	$ 41,117	$ 257,116	$ 122,902	$ 16,122	$ 442,034	$ 437,257	$ 397,595
Weighted Average common shares outstanding — basic (in shares)									111,409	111,026	110,626
Net effect of dilutive securities:
Contingently issuable performance shares and restricted stock units									637	526	552
Weighted average common shares outstanding — diluted (in shares)									112,046	111,552	111,178
Earnings per average common share attributable to common shareholders — basic (in dollars per share)	$ 0.48	$ 2.36	$ 1.09	$ 0.04	$ 0.37	$ 2.32	$ 1.11	$ 0.15	$ 3.97	$ 3.94	$ 3.59
Earnings per average common share attributable to common shareholders — diluted (in dollars per share)	$ 0.47	$ 2.35	$ 1.08	$ 0.04	$ 0.37	$ 2.30	$ 1.10	$ 0.14	$ 3.95	$ 3.92	$ 3.58