Resale and Repurchase Agreements Resale and Repurchase Agreements (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Balance Sheet Offsetting [Line Items]
Securities purchased under agreements to resell	$ 700,000	$ 700,000
Resale agreement [Member]
Balance Sheet Offsetting [Line Items]
Securities Purchased under Agreements to Resell, Gross	1,000,000	1,150,000
Securities Purchased under Agreements to Resell, Liability	(300,000)	(450,000)
Securities purchased under agreements to resell	700,000	700,000
Securities Purchased under Agreements to Resell, Not Subject to Master Netting Arrangement	0	0
Securities Purchased under Agreements to Resell, Collateral, Obligation to Return Securities	(700,000)	(700,000)
Securities Purchased under Agreements to Resell, Amount Offset Against Collateral	0	0
Repurchase agreement [Member]
Balance Sheet Offsetting [Line Items]
Securities Sold under Agreements to Repurchase, Gross	1,760,376	2,393,219
Securities Sold under Agreements to Repurchase, Asset	(300,000)	(450,000)
Securities Sold under Agreements to Repurchase	1,460,376	1,943,219
Securities Sold under Agreements to Repurchase, Not Subject to Master Netting Arrangement	0	0
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Securities	(1,460,376)	(1,943,219)
Securities Sold under Agreements to Repurchase, Amount Offset Against Collateral	$ 0	$ 0