Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income attributable to common shareholders	$ 21,629	$ 276,072	$ 167,443	$ 23,312	$ 53,246	$ 263,027	$ 121,308	$ 4,453	$ 488,456	$ 442,034	$ 437,257
Weighted Average common shares outstanding — basic (in shares)									111,839	111,409	111,026
Net effect of dilutive securities:
Contingently issuable performance shares and restricted stock units									528	637	526
Weighted average common shares outstanding — diluted (in shares)									112,367	112,046	111,552
Earnings per average common share attributable to common shareholders — basic (in dollars per share)	$ 0.19	$ 2.47	$ 1.50	$ 0.21	$ 0.48	$ 2.36	$ 1.09	$ 0.04	$ 4.37	$ 3.97	$ 3.94
Earnings per average common share attributable to common shareholders — diluted (in dollars per share)	$ 0.19	$ 2.46	$ 1.49	$ 0.21	$ 0.47	$ 2.35	$ 1.08	$ 0.04	$ 4.35	$ 3.95	$ 3.92