Income Taxes (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating Loss Carryforwards [Line Items]
Total income tax expense (benefit)	$ 177,288,000	$ 101,077,000	$ 498,752,000	$ 345,881,000
Document Period End Date			Sep. 30, 2017
Effective income tax rate	31.30%	27.60%	32.70%	35.00%
Available-for-sale Securities, Income Tax Expense on Change in Unrealized Holding Gain (Loss)	$ (6,400,000.0)		$ (6,500,000.0)	$ 3,300,000