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Shareholders' Equity, Dividends Restrictions and Other Regulatory Matters (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 USD ($) vote Rate shares | Dec. 31, 2016 USD ($) Rate shares | Dec. 31, 2015 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |
Capital required for common equity Tier 1 capital to risk-weighted assets | 4.50% |  |
Dividends from subsidiaries | $ 50,424 |  |
Tier 1 Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets | 6.00% |  |
Capital Required for Capital Adequacy to Risk Weighted Assets | 8.00% |  |
Leverage Capital Required Ratio To Tangible Assets | 4.00% |  |
Tier 2 capital | $ 0 | $ 3,000 |
Subordinated Debt, Annual Tier 2 Discount Percentage | Rate | 0.00% | 20.00% |
Maximum Undivided Profits Elgible For Dividend Payment Without Changing Well Capitalized Status | $ 1,074,000 |  |
Average Required Maintence Of Non Interst Bearing Reserve Balance | $ 625,700 |  |
Capital Conservation Buffer, Annual Percentage Increase | 0.625% |  |
Capital Conservation Buffer | 1.25% | 0.625% |
Fully Phased In 2019 Capital Conservation Buffer | 2.50% |  |
Common Class A [Member] |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |
Votes per share of common stock | vote | 1 |  |
Stock Repurchase Program, Number of Shares Authorized to be Repurchased | shares | 800,000 | 200,000 |
Stock Repurchased During Period, Shares | shares | 0 | 0 |
Class B Common Stock |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |
Votes per share of common stock | vote | 16 |  |
Stock Repurchased and Retired During Period, Shares | shares | 0 | 0 |
Parent |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |
Dividends from subsidiaries |  | $ 90,055 | $ 75,000
BancShares [Member] |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |
Tier 1 Capital for Capital Adequacy | $ 3,287,364 | 2,995,557 |
Capital Required for Capital Adequacy | 3,626,789 | 3,339,986 |
Leverage Capital Required for Capital Adequacy | $ 3,287,364 | $ 2,995,557 |
Tier 1 Risk Based Capital to Risk Weighted Assets | 12.88% | 12.42% |
Capital to Risk Weighted Assets | 14.21% | 13.85% |
Leverage Capital to Average Assets | 9.47% | 9.05% |
Tier 1 Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets | 8.00% | 8.00% |
Common equity Tier 1 | $ 3,287,364 | $ 2,995,557 |
Common equity Tier 1 to risk-weighted assets | 12.88% | 12.42% |
Common equity Tier 1 required to be well capitalized to risk-weighted assets | 6.50% | 6.50% |
Capital Required to be Well Capitalized | 10.00% | 10.00% |
Leverage Capital Required to be Well Capitalized to Average Assets | 5.00% | 5.00% |
Capital Conservation Buffer | 6.21% |  |
FCB [Member] |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |
Tier 1 Capital for Capital Adequacy | $ 3,189,709 | $ 2,942,829 |
Capital Required for Capital Adequacy | 3,422,634 | 3,172,757 |
Leverage Capital Required for Capital Adequacy | $ 3,189,709 | $ 2,942,829 |
Tier 1 Risk Based Capital to Risk Weighted Assets | 12.54% | 12.25% |
Capital to Risk Weighted Assets | 13.46% | 13.21% |
Leverage Capital to Average Assets | 9.22% | 8.94% |
Tier 1 Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets | 8.00% | 8.00% |
Common equity Tier 1 | $ 3,189,709 | $ 2,942,829 |
Common equity Tier 1 to risk-weighted assets | 12.54% | 12.25% |
Common equity Tier 1 required to be well capitalized to risk-weighted assets | 6.50% | 6.50% |
Capital Required to be Well Capitalized | 10.00% | 10.00% |
Leverage Capital Required to be Well Capitalized to Average Assets | 5.00% | 5.00% |
Capital Conservation Buffer | 5.46% |  |

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