Summary of Significant Accounting Policies Financial Impact of Adoption of New Accounting Standards (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Significant Accounting Policies [Line Items]
Income taxes				$ 9,215					$ 60,609	$ 55,956	$ 52,285
Net income	$ 31,821	$ 47,565	$ 38,822	$ 21,163	$ 29,203	$ 41,419	$ 35,813	$ 21,594	$ 139,371	$ 128,029	$ 123,160
Basic earnings per share (in dollars per share)				$ 0.38					$ 2.48	$ 2.24	$ 2.11
Diluted earnings per share (in dollars per share)				$ 0.37					$ 2.46	$ 2.22	$ 2.10
Net cash used by operating activities				$ (21,380)					$ 152,035	$ 165,079	$ 187,612
Net cash provided by financing activities				62,627					(44,084)	(84,658)	$ (121,261)
Additional paid-in-capital	159,045			148,562	149,895				159,045	149,895
Retained earnings	$ 607,560			524,419	$ 514,897				$ 607,560	$ 514,897
Scenario, Previously Reported
Significant Accounting Policies [Line Items]
Income taxes				10,780
Net income				$ 19,598
Basic earnings per share (in dollars per share)				$ 0.35
Diluted earnings per share (in dollars per share)				$ 0.35
Net cash used by operating activities				$ (22,945)
Net cash provided by financing activities				64,192
Additional paid-in-capital				150,127
Retained earnings				$ 522,854