Regulatory Capital - Schedule of Actual Capital Amounts and Ratios and Selected Minimum Regulatory Thresholds (Details) - USD ($) $ in Millions	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2016
Total risk-based capital:
Actual amount		$ 1,285.0	$ 1,112.5
Actual ratio (percent)		12.99%	12.76%
Adequately capitalized Basel III phase-in schedule, amount		$ 976.6	$ 806.5
Adequately capitalized Basel III phase-in schedule, ratio (percent)		9.875%	9.25%
Adequately capitalized Basel III fully phased-in, amount		$ 1,038.4	$ 915.5
Adequately capitalized Basel III fully phased-in, ratio (percent)		10.50%	10.50%
Well capitalized, amount		$ 989.0	$ 871.9
Well capitalized, ratio (percent)		10.00%	10.00%
Tier 1 risk-based capital:
Actual amount		$ 1,212.0	$ 1,040.3
Actual ratio (percent)		12.26%	11.93%
Adequately capitalized Basel III phase-in schedule, amount		$ 778.8	$ 632.1
Adequately capitalized Basel III phase-in schedule, ratio (percent)		7.875%	7.25%
Adequately capitalized Basel III fully phased-in, amount		$ 840.6	$ 741.1
Adequately capitalized Basel III fully phased-in, ratio (percent)		8.50%	8.50%
Well capitalized, amount		$ 791.2	$ 697.5
Well capitalized, ratio (percent)		8.00%	8.00%
Common Equity Tier One Capital [Abstract]
Actual amount		$ 1,127.8	$ 962.4
Actual ratio (percent)		11.40%	11.04%
Adequately capitalized Basel III phase-in schedule, amount		$ 630.5	$ 501.3
Adequately capitalized Basel III phase-in schedule, ratio (percent)		6.375%	5.75%
Adequately capitalized Basel III fully phased-in, amount		$ 692.3	$ 610.3
Adequately capitalized Basel III fully phased-in, ratio (percent)		7.00%	7.00%
Well capitalized, amount		$ 642.8	$ 566.7
Well capitalized, ratio (percent)		6.50%	6.50%
Leverage capital ratio:
Actual amount		$ 1,212.0	$ 1,040.3
Actual ratio (percent)		9.47%	8.86%
Adequately capitalized, amount		$ 511.9	$ 469.9
Adequately capitalized, ratio (percent)		4.00%	4.00%
Well capitalized, amount		$ 639.9	$ 587.4
Well capitalized, ratio (percent)		5.00%	5.00%
Required Capital Conservation Buffer, phase in amount				0.625%
Capital Conservation Buffer		4.99%
FIB
Total risk-based capital:
Actual amount		$ 1,184.5	$ 1,066.6
Actual ratio (percent)		12.01%	12.29%
Adequately capitalized Basel III phase-in schedule, amount		$ 973.7	$ 802.7
Adequately capitalized Basel III phase-in schedule, ratio (percent)		9.875%	9.25%
Adequately capitalized Basel III fully phased-in, amount		$ 1,035.3	$ 911.2
Adequately capitalized Basel III fully phased-in, ratio (percent)		10.50%	10.50%
Well capitalized, amount		$ 986.0	$ 867.8
Well capitalized, ratio (percent)		10.00%	10.00%
Tier 1 risk-based capital:
Actual amount		$ 1,111.6	$ 994.4
Actual ratio (percent)		11.27%	11.46%
Adequately capitalized Basel III phase-in schedule, amount		$ 776.5	$ 629.1
Adequately capitalized Basel III phase-in schedule, ratio (percent)		7.875%	7.25%
Adequately capitalized Basel III fully phased-in, amount		$ 838.1	$ 737.6
Adequately capitalized Basel III fully phased-in, ratio (percent)		8.50%	8.50%
Well capitalized, amount		$ 788.8	$ 694.2
Well capitalized, ratio (percent)		8.00%	8.00%
Common Equity Tier One Capital [Abstract]
Actual amount		$ 1,111.6	$ 994.4
Actual ratio (percent)		11.27%	11.46%
Adequately capitalized Basel III phase-in schedule, amount		$ 628.6	$ 499.0
Adequately capitalized Basel III phase-in schedule, ratio (percent)		6.375%	5.75%
Adequately capitalized Basel III fully phased-in, amount		$ 690.2	$ 607.4
Adequately capitalized Basel III fully phased-in, ratio (percent)		7.00%	7.00%
Well capitalized, amount		$ 640.9	$ 564.1
Well capitalized, ratio (percent)		6.50%	6.50%
Leverage capital ratio:
Actual amount		$ 1,111.6	$ 994.4
Actual ratio (percent)		8.97%	8.48%
Adequately capitalized, amount		$ 495.9	$ 469.1
Adequately capitalized, ratio (percent)		4.00%	4.00%
Well capitalized, amount		$ 619.8	$ 586.3
Well capitalized, ratio (percent)		5.00%	5.00%
Capital Conservation Buffer			4.01%
Subsequent Event
Compliance with Regulatory Capital Requirements under Banking Regulations
Tier One Common Equity Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets, excluding buffer	4.50%
Required Capital Conservation Buffer	2.50%
Total risk-based capital:
Adequately capitalized Basel III fully phased-in, ratio (percent)	10.50%
Tier 1 risk-based capital:
Adequately capitalized Basel III fully phased-in, ratio (percent)	8.50%
Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets, excluding buffer	8.00%
Common Equity Tier One Capital [Abstract]
Adequately capitalized Basel III fully phased-in, ratio (percent)	7.00%
Tier One Capital Required for Capital Adequacy under Basel III Fully Phased-in to Risk Weighted Assets, excluding buffer	6.00%
Leverage capital ratio:
Adequately capitalized, ratio (percent)	4.00%