Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income attributable to common shareholders	$ 26,076	$ 315,012	$ 166,738	$ 3,221	$ 21,629	$ 276,072	$ 167,443	$ 23,312	$ 511,047	$ 488,456	$ 442,034
Weighted Average common shares outstanding — basic (in shares)									112,129	111,839	111,409
Net effect of dilutive securities:
Contingently issuable performance shares and restricted stock units									421	528	637
Weighted average common shares outstanding — diluted (in shares)									112,550	112,367	112,046
Earnings per average common share attributable to common shareholders — basic (in dollars per share)	$ 0.23	$ 2.81	$ 1.49	$ 0.03	$ 0.19	$ 2.47	$ 1.50	$ 0.21	$ 4.56	$ 4.37	$ 3.97
Earnings per average common share attributable to common shareholders — diluted (in dollars per share)	$ 0.23	$ 2.80	$ 1.48	$ 0.03	$ 0.19	$ 2.46	$ 1.49	$ 0.21	$ 4.54	$ 4.35	$ 3.95