Short and Long-Term Funding FHLB Maturity Schedule (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
FHLB_Maturity_ [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	$ 3,332,655	$ 3,184,168
FHLB Weighted Average Coupon Rate	2.10%	1.26%
FHLB Maturity within One Year [Domain]
FHLB_Maturity_ [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	$ 1,371,059	$ 2,434,000
FHLB Weighted Average Coupon Rate	1.76%	1.26%
FHLB Maturity One Year to Two Years [Domain]
FHLB_Maturity_ [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	$ 1,662,019	$ 750,013
FHLB Weighted Average Coupon Rate	2.29%	1.23%
FHLB Maturity Over Two Years to Three Years [Domain]
FHLB_Maturity_ [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	$ 216,289	$ 155
FHLB Weighted Average Coupon Rate	2.53%	4.91%
FHLB Maturity Over Three Years [Domain]
FHLB_Maturity_ [Line Items]
Federal Home Loan Bank, Advances, Branch of FHLB Bank, Amount of Advances	$ 83,289	$ 0
FHLB Weighted Average Coupon Rate	2.62%	0.00%