Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]
Net income attributable to common shareholders	$ 41,117	$ 257,116	$ 122,902	$ 16,122	$ 5,410	$ 243,961	$ 132,458	$ 15,766	$ 437,257	$ 397,595	$ 406,074
Weighted Average common shares outstanding — basic (in shares)									111,026	110,626	109,984
Net effect of dilutive securities:
Contingently issuable performance shares and restricted stock units									526	552	822
Weighted average common shares outstanding — diluted (in shares)									111,552	111,178	110,806
Earnings per average common share attributable to common shareholders — basic (in dollars per share)	$ 0.37	$ 2.32	$ 1.11	$ 0.15	$ 0.05	$ 2.20	$ 1.20	$ 0.14	$ 3.94	$ 3.59	$ 3.69
Earnings per average common share attributable to common shareholders — diluted (in dollars per share)	$ 0.37	$ 2.30	$ 1.10	$ 0.14	$ 0.05	$ 2.20	$ 1.19	$ 0.14	$ 3.92	$ 3.58	$ 3.66