- SOI Proceeds from sales (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Proceeds from sales	$ 3,933,887	$ 4,541,303	$ 4,187,547
Gross realized gains from sales	64,885	84,305	47,965
Gross realized losses from sales	$ 30,991	$ 16,858	$ 6,476