Deposits - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Banking and Thrift [Abstract]
Stable interest-bearing deposits, total	$ 5,500.0		$ 5,500.0
Brokered deposit placement fee	2.5	$ 2.6	6.8	$ 7.8
Third party broker fees paid	4.4	$ 1.1	9.7	$ 4.0
Deposits exceeding FDIC insurance limits	422.4		422.4		$ 304.5
Accrued interest on deposits	$ 28.2		$ 28.2		$ 18.9