Loans - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Eligible borrowing capacity in FHLB advances	$ 2,600,000	$ 2,000,000
FRBs Discount Window Primary Credit Program
Accounts, Notes, Loans and Financing Receivable [Line Items]
Eligible borrowing capacity through FRB's Discount Window Primary Credit Program	$ 843,618	$ 629,699