Securities Purchased under Resale Agreements and Sold under Repurchase Agreements (Balance Sheet Offsetting) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets, Resale Agreements
Gross Amounts of Recognized Assets	$ 1,450,000	$ 1,450,000
Gross Amounts Offset on the Consolidated Balance Sheet	(400,000)	(400,000)
Net Amounts of Assets Presented on the Consolidated Balance Sheet	1,050,000	1,050,000
Gross Amounts Not Offset on the Consolidated Balance Sheet
Financial Instruments	0	0
Collateral Received	(1,037,413)	(1,045,696)
Net Amount	12,587	4,304
Liabilities, Repurchase Agreements
Gross Amounts of Recognized Liabilities	450,000	450,000
Gross Amounts Offset on the Consolidated Balance Sheet	(400,000)	(400,000)
Net Amounts of Liabilities Presented on the Consolidated Balance Sheet	50,000	50,000
Gross Amounts Not Offset on the Consolidated Balance Sheet
Financial Instruments	0	0
Collateral Pledged	(50,000)	(50,000)
Net Amount	$ 0	$ 0