Earnings Per Share - Computation Of Basic And Diluted Earnings Per Common Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Earnings Per Share [Abstract]
Net income from continuing operations	$ 25,309	$ 39,416	$ 35,279	$ 21,469	$ 23,377	$ 38,573	$ 31,624	$ 19,776	$ 121,473		$ 113,350		$ 121,278
Net income from discontinued operations	(24)	(51)	121	1,641	66	143	183	(33)	1,687		359		335
Net income	25,285	39,365	35,400	23,110	23,443	38,716	31,807	19,743	123,160		113,709		121,613
Income allocated to participating securities									(1,075)		(1,127)		(1,281)
Income allocated to participating securities									(1,069)		(1,122)		(1,299)
Net income available to common shareholders									122,085		112,582		120,332
Weighted average common shares outstanding -- basic									57,730		57,925		57,467
Basic earnings per share, Continuing operations (in dollars per share)	$ 0.44	$ 0.67	$ 0.61	$ 0.37	$ 0.40	$ 0.66	$ 0.54	$ 0.34	$ 2.08	[1]	$ 1.94	[1]	$ 2.09
Basic earnings per share, Discontinued operations (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ 0.03	$ 0.00	$ 0.00	$ 0.00	$ 0.00	$ 0.03	[1]	$ 0.00	[1]	$ 0.00
Basic earnings per share (in dollars per share)									$ 2.11		$ 1.94		$ 2.09
Net income available to common shareholders									$ 122,091		$ 112,587		$ 120,314
Diluted effect of stock options and PVRSUs									526		410		186
Weighted average commons shares outstanding -- diluted									58,256		58,335		57,653
Diluted earnings per share, Continuing operations (in dollars per share)	$ 0.43	$ 0.67	$ 0.60	$ 0.36	$ 0.40	$ 0.65	$ 0.54	$ 0.34	$ 2.07	[1]	$ 1.92	[1]	$ 2.08
Diluted earnings per share, Discontinued Operations (in dollars per share)	$ 0.00	$ 0.00	$ 0.00	$ 0.03	$ 0.00	$ 0.01	$ 0.00	$ 0.00	$ 0.03	[1]	$ 0.01	[1]	$ 0.01
Diluted earnings per share (in dollars per share)									$ 2.10		$ 1.93		$ 2.09

[1] The sum of the earnings per share for the four quarters may differ from annual earnings per share due to the required method of computing the weighted average shares in interim periods.