Earnings Per Share - Computation Of Basic And Diluted Earnings Per Common 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	$ 31,471	$ 79,959	$ 79,839	$ 25,086	$ (9,816)	$ 57,189	$ 50,685	$ 24,269	$ 216,355	$ 122,327	$ 106,712
Income allocated to participating securities, basic									(1,248)	(845)	(747)
Income allocated to participating securities, diluted									(1,241)	(842)	(744)
Net income available to common shareholders, basic									$ 215,107	$ 121,482	$ 105,965
Weighted average common shares outstanding -- basic (in shares)									56,130	56,114	55,872
Basic earnings per share (in dollars per share)	$ 0.56	$ 1.42	$ 1.41	$ 0.44	$ (0.17)	$ 1.01	$ 0.90	$ 0.43	$ 3.83	$ 2.16	$ 1.90
Net income available to common shareholders, diluted									$ 215,114	$ 121,485	$ 105,968
Diluted effect of stock options and PVRSUs (in shares)									471	412	283
Weighted average commons shares outstanding -- diluted (in shares)									56,601	56,526	56,155
Diluted earnings per share (in dollars per share)	$ 0.56	$ 1.41	$ 1.40	$ 0.44	$ (0.17)	$ 1.01	$ 0.89	$ 0.43	$ 3.80	$ 2.15	$ 1.89