Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Total risk-based capital:
Actual amount	$ 1,112.5	$ 974.5
Actual ratio (percent)	12.76%	15.13%
Adequately capitalized Basel III phase-in schedule, amount	$ 806.5	$ 555.7
Adequately capitalized Basel III phase-in schedule, ratio (percent)	9.25%	8.63%
Adequately capitalized Basel III fully phased-in, amount	$ 915.5	$ 676.1
Adequately capitalized Basel III fully phased-in, ratio (percent)	10.50%	10.50%
Well capitalized, amount	$ 871.9	$ 643.9
Well capitalized, ratio (percent)	10.00%	10.00%
Tier 1 risk-based capital:
Actual amount	$ 1,040.3	$ 894.3
Actual ratio (percent)	11.93%	13.89%
Adequately capitalized Basel III phase-in schedule, amount	$ 632.1	$ 426.9
Adequately capitalized Basel III phase-in schedule, ratio (percent)	7.25%	6.63%
Adequately capitalized Basel III fully phased-in, amount	$ 741.1	$ 547.3
Adequately capitalized Basel III fully phased-in, ratio (percent)	8.50%	8.50%
Well capitalized, amount	$ 697.5	$ 515.1
Well capitalized, ratio (percent)	8.00%	8.00%
Common Equity Tier One Capital [Abstract]
Actual amount	$ 962.4	$ 814.3
Actual ratio (percent)	11.04%	12.65%
Adequately capitalized Basel III phase-in schedule, amount	$ 501.3	$ 330.3
Adequately capitalized Basel III phase-in schedule, ratio (percent)	5.75%	5.13%
Adequately capitalized Basel III fully phased-in, amount	$ 610.3	$ 450.7
Adequately capitalized Basel III fully phased-in, ratio (percent)	7.00%	7.00%
Well capitalized, amount	$ 566.7	$ 418.5
Well capitalized, ratio (percent)	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 1,040.3	$ 894.3
Actual ratio (percent)	8.86%	10.11%
Adequately capitalized, amount	$ 469.9	$ 353.8
Adequately capitalized, ratio (percent)	4.00%	4.00%
Well capitalized, amount	$ 587.4	$ 442.3
Well capitalized, ratio (percent)	5.00%	5.00%
Capital Conservation Buffer	4.76%
FIB
Total risk-based capital:
Actual amount	$ 1,066.6	$ 842.0
Actual ratio (percent)	12.29%	13.13%
Adequately capitalized Basel III phase-in schedule, amount	$ 802.7	$ 553.5
Adequately capitalized Basel III phase-in schedule, ratio (percent)	9.25%	8.63%
Adequately capitalized Basel III fully phased-in, amount	$ 911.2	$ 673.4
Adequately capitalized Basel III fully phased-in, ratio (percent)	10.50%	10.50%
Well capitalized, amount	$ 867.8	$ 641.3
Well capitalized, ratio (percent)	10.00%	10.00%
Tier 1 risk-based capital:
Actual amount	$ 994.4	$ 765.8
Actual ratio (percent)	11.46%	11.94%
Adequately capitalized Basel III phase-in schedule, amount	$ 629.1	$ 425.2
Adequately capitalized Basel III phase-in schedule, ratio (percent)	7.25%	6.63%
Adequately capitalized Basel III fully phased-in, amount	$ 737.6	$ 545.1
Adequately capitalized Basel III fully phased-in, ratio (percent)	8.50%	8.50%
Well capitalized, amount	$ 694.2	$ 513.1
Well capitalized, ratio (percent)	8.00%	8.00%
Common Equity Tier One Capital [Abstract]
Actual amount	$ 994.4	$ 765.8
Actual ratio (percent)	11.46%	11.94%
Adequately capitalized Basel III phase-in schedule, amount	$ 499.0	$ 329.0
Adequately capitalized Basel III phase-in schedule, ratio (percent)	5.75%	5.13%
Adequately capitalized Basel III fully phased-in, amount	$ 607.4	$ 448.9
Adequately capitalized Basel III fully phased-in, ratio (percent)	7.00%	7.00%
Well capitalized, amount	$ 564.1	$ 416.9
Well capitalized, ratio (percent)	6.50%	6.50%
Leverage capital ratio:
Actual amount	$ 994.4	$ 765.8
Actual ratio (percent)	8.48%	8.69%
Adequately capitalized, amount	$ 469.1	$ 352.3
Adequately capitalized, ratio (percent)	4.00%	4.00%
Well capitalized, amount	$ 586.3	$ 440.4
Well capitalized, ratio (percent)	5.00%	5.00%
Capital Conservation Buffer		4.29%