Company Information and Significant Accounting Policies - Effect of New Accounting Pronouncements (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Consolidated Statements of Income:
Income taxes	$ 16,788	$ 9,215	$ 17,066	$ 26,003	$ 26,506
Net income	$ 38,822	$ 21,163	$ 35,813	$ 59,985	$ 57,407
Basic earnings per share (in dollars per share)	$ 0.69	$ 0.38	$ 0.62	$ 1.06	$ 1.00
Diluted earnings per share (in dollars per share)	$ 0.68	$ 0.37	$ 0.62	$ 1.06	$ 0.99
Consolidated Statements of Cash Flows:
Net cash used by operating activities		$ (21,380)		$ 27,861	$ 42,543
Net cash provided by financing activities		62,627		40,090	$ (16,657)
Consolidated Balance Sheets:
Additional paid-in-capital	$ 152,596	148,562		152,596		$ 149,895
Retained earnings	$ 551,749	524,419		$ 551,749		$ 514,897
Scenario, Previously Reported
Consolidated Statements of Income:
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
Consolidated Statements of Cash Flows:
Net cash used by operating activities		$ (22,945)
Net cash provided by financing activities		64,192
Consolidated Balance Sheets:
Additional paid-in-capital		150,127
Retained earnings		$ 522,854