Securities Purchased under Resale Agreements and Sold under Repurchase Agreements (Resale Agreements and Repurchase Agreements) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Resale agreements
Gross resale agreements	$ 1,435,000		$ 1,435,000		$ 1,450,000
Weighted-average yields	2.63%	2.29%	2.59%	2.13%
Repurchase agreements
Gross repurchase agreements	$ 450,000		$ 450,000		$ 450,000
Weighted-average yields	4.65%	3.56%	4.36%	3.42%