Shareholders' Equity, Dividends Restrictions and Other Regulatory Matters (Details) $ / shares in Units, $ in Thousands		12 Months Ended
	Mar. 12, 2020 shares	Dec. 31, 2020 USD ($) vote $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Dec. 31, 2018 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		$ 229,685
Tier 1 Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets		0.0600
Capital Required for Capital Adequacy to Risk Weighted Assets		0.0800
Leverage Capital Required Ratio To Tangible Assets		0.0400
Stock repurchases during period		$ 333,800	$ 450,800
Price of repurchased shares (usd per share) | $ / shares		$ 410.48	$ 451.33
Outstanding Shares Repurchased, Percent			8.40%	9.40%
Maximum Undivided Profits Elgible For Dividend Payment Without Changing Well Capitalized Status		$ 1,700,000
Average Required Maintence Of Non Interst Bearing Reserve Balance		$ 115,200
Capital Conservation Buffer		2.50%
Net proceeds from preferred stock issuance		$ 339,937	$ 0	$ 0
Non-Cumulative Perpetual Preferred Stock, Series A [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred Stock, dividend rate, percentage		5.375%
Preferred stock, par value (in dollars per share) | $ / shares		$ 0.01
Preferred Stock, Liquidation Preference Per Share | $ / shares		1,000
Fixed-to-Floating Rate Subordinated Notes Due 2030 [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred Stock, Liquidation Preference Per Share | $ / shares		$ 25
Qualifying Subordinated Debentures [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 2 capital		$ 377,500	32,500
Common Class A [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Votes per share of common stock | vote		1
Stock repurchases during period		$ 333,755	$ 450,880	165,338
Stock Repurchased and Retired During Period, Shares | shares		813,090	998,910
Class B Common Stock
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Votes per share of common stock | vote		16
Depositary Shares [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock Issued During Period, Shares, New Issues | shares	13,800,000
Net proceeds from preferred stock issuance		$ 345,000
Trusted Preferred Capital Securities [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 2 capital		128,500	$ 128,500
Parent
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Dividends from subsidiaries			149,819	242,900
Net proceeds from preferred stock issuance		339,937	0	$ 0
BancShares
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Capital for Capital Adequacy		4,577,212	3,731,501
Capital Required for Capital Adequacy		3,516,149	3,344,305
Leverage Capital Required for Capital Adequacy		$ 3,856,086	$ 3,344,305
Tier 1 Risk Based Capital to Risk Weighted Assets		0.1381	0.1212
Capital to Risk Weighted Assets		0.1061	0.1086
Leverage Capital to Average Assets		0.0786	0.0881
Tier 1 Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets		0.1000
Common equity Tier 1		$ 3,856,086	$ 3,344,305
Common equity Tier 1 to risk-weighted assets		0.1163	0.1086
Common equity Tier 1 required to be well capitalized to risk-weighted assets		8.00%
Capital Required to be Well Capitalized		0.0650
Leverage Capital Required to be Well Capitalized to Average Assets		0.0500
Capital Conservation Buffer			5.63%
FCB [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Capital for Capital Adequacy		$ 4,543,496	$ 3,837,670
Capital Required for Capital Adequacy		4,276,870	3,554,974
Leverage Capital Required for Capital Adequacy		$ 4,276,870	$ 3,554,974
Tier 1 Risk Based Capital to Risk Weighted Assets		0.1372	0.1246
Capital to Risk Weighted Assets		0.1292	0.1154
Leverage Capital to Average Assets		0.0872	0.0938
Tier 1 Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets		0.1000
Common equity Tier 1		$ 4,276,870	$ 3,554,974
Common equity Tier 1 to risk-weighted assets		0.1292	0.1154
Common equity Tier 1 required to be well capitalized to risk-weighted assets		8.00%
Capital Required to be Well Capitalized		0.0650
Leverage Capital Required to be Well Capitalized to Average Assets		0.0500
Capital Conservation Buffer			5.72%