Short-term Funding Short-term Funding, Repurchase Agreements (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Remaining Contractual Maturity of the Agreements
Repurchase agreements	$ 182,865	$ 300,197
Agency mortgage-related securities
Remaining Contractual Maturity of the Agreements
Repurchase agreements	182,865	300,197
Overnight and Continuous
Remaining Contractual Maturity of the Agreements
Repurchase agreements	182,865	300,197
Overnight and Continuous | Agency mortgage-related securities
Remaining Contractual Maturity of the Agreements
Repurchase agreements	182,865	300,197
Up to 30 days
Remaining Contractual Maturity of the Agreements
Repurchase agreements	0	0
Up to 30 days | Agency mortgage-related securities
Remaining Contractual Maturity of the Agreements
Repurchase agreements	0	0
30-90 days
Remaining Contractual Maturity of the Agreements
Repurchase agreements	0	0
30-90 days | Agency mortgage-related securities
Remaining Contractual Maturity of the Agreements
Repurchase agreements	0	0
Greater than 90 days
Remaining Contractual Maturity of the Agreements
Repurchase agreements	0	0
Greater than 90 days | Agency mortgage-related securities
Remaining Contractual Maturity of the Agreements
Repurchase agreements	$ 0	$ 0