Securities Purchased under Resale Agreements and Sold under Repurchase Agreements (Repurchase Agreements) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amount of securities sold under repurchase agreements
Gross repurchase agreements	$ 450,000	$ 450,000
Weighted average interest rates (as a percent)	3.65%	3.15%
Repurchase agreements’ extinguishment costs	$ 0	$ 0	$ 21,818
Maturities in 2018 through 2022	150,000
Maturities in 2023 and thereafter	$ 300,000
Repurchase Agreements
Amount of securities sold under repurchase agreements
Repurchase agreements extinguished			$ 545,000