Significant Accounting Policies Significant Accounting Policies - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income Tax Expense (Benefit)	$ (13,838)	$ (8,378)	$ (25,239)	$ (17,770)
Net Income (Loss) Available to Common Stockholders, Basic	$ 83,538	$ 69,492	$ 166,463	$ 136,292
Earnings Per Share, Diluted	$ 1.29	$ 1.11	$ 2.57	$ 2.19
Adjustments for New Accounting Pronouncement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Income Tax Expense (Benefit)		$ 28		$ 65
Net Income (Loss) Available to Common Stockholders, Basic		$ 28		$ 65
Earnings Per Share, Diluted				$ 0.01