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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% |  |  |

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- Definition The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958570-112826 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 + Details
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- Definition The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
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- Definition The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
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- Definition Tier 1 capital divided by risk weighted assets as defined by regulations. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
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