Consolidated Statements of Changes In Shareholders' Equity - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Class A Common Stock	Class B Common Stock	Common Stock	Common Stock Class A Common Stock	Common Stock Class B Common Stock	Preferred Stock	Surplus	Surplus Class A Common Stock	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings Class A Common Stock	Retained Earnings Class B Common Stock	Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Cumulative Effect, Period of Adoption, Adjustment
Beginning balance at Dec. 31, 2017	$ 3,334,064					$ 11,005	$ 1,005	$ 0	$ 658,918		$ 2,785,430				$ (122,294)
Beginning balance (Cumulative effect of adoption of ASU 2016-01) at Dec. 31, 2017												$ 18,715				$ (18,715)
Beginning balance (Cumulative effect of adoption of ASU 2018-02) at Dec. 31, 2017												31,336				$ (31,336)
Beginning balance (Cumulative effect of adoption of ASC 326) at Dec. 31, 2017		$ 36,943										$ 36,943
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	400,313										400,313
Other comprehensive income (loss), net of tax	(62,842)														(62,842)
Stock repurchases during period			$ 165,338			382				$ 164,956
Cash dividends	(1,458)		(15,785)								(1,458)		$ (15,785)
Ending balance at Dec. 31, 2018	3,488,954					10,623	1,005	0	493,962		3,218,551				(235,187)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	457,371										457,371
Other comprehensive income (loss), net of tax	108,464														108,464
Stock repurchases during period	450,800		450,880			999				449,881
Cash dividends			(16,117)	$ (1,608)		0	0		0				(16,117)	$ (1,608)	0
Ending balance at Dec. 31, 2019	3,586,184				$ 1,005	9,624		0	44,081		3,658,197				(126,723)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	491,723										491,723
Other comprehensive income (loss), net of tax	138,986														138,986
Issuance of preferred stock	339,937							339,937
Stock repurchases during period	333,800		333,755			813				$ 44,081			288,861
Cash dividends			$ (15,010)	$ (1,678)									$ (15,010)	$ (1,678)
Preferred stock dividends declared	14,062										14,062
Ending balance at Dec. 31, 2020	$ 4,229,268					$ 8,811	$ 1,005	$ 339,937	$ 0		$ 3,867,252				$ 12,263