Securities Purchased under Resale Agreements and Sold under Repurchase Agreements (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Resale agreements
Gross resale agreements	$ 1,650,000	$ 2,100,000
Weighted average interest rates (as a percent)	2.30%	1.84%
Repurchase agreements
Gross repurchase agreements	$ 450,000	$ 450,000
Weighted average interest rates (as a percent)	3.56%	3.15%