Company Information and Significant Accounting Policies Financial Impact of Adoption of New Accounting Standards (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Significant Accounting Policies [Line Items]
Income taxes	$ 22,635	$ 9,215	$ 20,849	$ 48,638	$ 47,355
Net income	$ 47,565	$ 21,163	$ 41,419	$ 107,550	$ 98,826
Basic earnings per share (in dollars per share)	$ 0.85	$ 0.38	$ 0.72	$ 1.91	$ 1.72
Diluted earnings per share (in dollars per share)	$ 0.84	$ 0.37	$ 0.72	$ 1.90	$ 1.71
Net cash used by operating activities		$ (21,380)		$ 88,409	$ 103,658
Net cash provided by financing activities		62,627		(10,152)	$ (54,397)
Additional paid-in-capital	$ 155,393	148,562		155,393		$ 149,895
Retained earnings	$ 587,837	524,419		$ 587,837		$ 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