Shareholders' Equity, Dividends Restrictions and Other Regulatory Matters (Details) $ / shares in Units, $ in Thousands	2 Months Ended	12 Months Ended
	Feb. 14, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) votes / shares $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($)	Jan. 28, 2020 shares	Jan. 01, 2018	Jan. 01, 2016
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		$ 149,819
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		$ 32,500	$ 20,000
Stock repurchases during period		$ 450,800	$ 165,300
Price of repurchased shares (usd per share) | $ / shares		$ 451.33	$ 432.78
Outstanding Shares Repurchased, Percent			9.40%	3.50%
Maximum Undivided Profits Elgible For Dividend Payment Without Changing Well Capitalized Status		$ 651,700
Average Required Maintence Of Non Interst Bearing Reserve Balance		$ 730,700
Capital Conservation Buffer, Annual Percentage Increase							0.625%
Capital Conservation Buffer		2.50%				1.875%	0.625%
Fully Phased In 2019 Capital Conservation Buffer		2.50%
Subsequent Event
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock repurchases during period	$ 63,800
Price of repurchased shares (usd per share) | $ / shares	$ 527.27
Common Class A [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Votes per share of common stock | votes / shares		1
Stock repurchases (shares) | shares		998,910	382,000
Stock repurchases during period		$ 450,880	$ 165,338
Stock Repurchased and Retired During Period, Shares | shares		998,910	382,000
Common Class A [Member] | Subsequent Event
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock Repurchase Program, Number of Shares Authorized to be Repurchased | shares					500,000
Stock repurchases (shares) | shares	120,990
Class B Common Stock
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Votes per share of common stock | votes / shares		16
Parent
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Dividends from subsidiaries			$ 242,910	$ 50,400
BancShares [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Capital for Capital Adequacy		$ 3,344,305	3,463,307
Capital Required for Capital Adequacy		3,731,501	3,826,626
Leverage Capital Required for Capital Adequacy		$ 3,344,305	$ 3,463,307
Tier 1 Risk Based Capital to Risk Weighted Assets		10.86%	12.67%
Capital to Risk Weighted Assets		12.12%	13.99%
Leverage Capital to Average Assets		8.81%	9.77%
Tier 1 Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets		8.00%
Common equity Tier 1		$ 3,344,305	$ 3,463,307
Common equity Tier 1 to risk-weighted assets		10.86%	12.67%
Common equity Tier 1 required to be well capitalized to risk-weighted assets		6.50%
Capital Required to be Well Capitalized		10.00%
Leverage Capital Required to be Well Capitalized to Average Assets		5.00%
Capital Conservation Buffer			4.12%
FCB [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Capital for Capital Adequacy		$ 3,554,974	$ 3,315,742
Capital Required for Capital Adequacy		3,837,670	3,574,561
Leverage Capital Required for Capital Adequacy		$ 3,554,974	$ 3,315,742
Tier 1 Risk Based Capital to Risk Weighted Assets		11.54%	12.17%
Capital to Risk Weighted Assets		12.46%	13.12%
Leverage Capital to Average Assets		9.38%	9.39%
Tier 1 Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets		8.00%
Common equity Tier 1		$ 3,554,974	$ 3,315,742
Common equity Tier 1 to risk-weighted assets		11.54%	12.17%
Common equity Tier 1 required to be well capitalized to risk-weighted assets		6.50%
Capital Required to be Well Capitalized		10.00%
Leverage Capital Required to be Well Capitalized to Average Assets		5.00%
Capital Conservation Buffer			4.46%