Summary of Investments - AFS Proceeds from sales (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Investments, Debt and Equity Securities [Abstract]
Proceeds from sales	$ 433,643	$ 339,051	$ 1,911,262	$ 2,810,599
Gross realized gains from sales	5,887	8,512	30,613	29,433
Gross realized losses from sales	$ 3,650	$ 2,993	$ 19,722	$ 24,330