Shareholders' Equity, Dividends Restrictions and Other Regulatory Matters (Details) $ in Thousands	12 Months Ended
	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%