Other Borrowings - Short-term FHLB Advances - (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
FHLB Advances
Outstanding advances	$ 100,000,000	$ 0
FHLB Advances
FHLB Advances
Loans pledged via a blanket lien to the FHLB for advances and letters of credit	2,600,000,000
Total borrowing capacity at FHLB	1,900,000,000
Letter of credit borrowed	6,300,000
Unused net credit available with the FHLB	$ 1,900,000,000
Long-term Debt, Weighted Average Interest Rate, at Point in Time	1.48%
Long-term Debt, Weighted Average Interest Rate, over Time	0.97%