Consolidated Statements of Changes In Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Adoption of ASC 326	Class A Common Stock	Class B Common Stock	Common Stock Class A Common Stock	Common Stock Class B Common Stock	Preferred Stock	Surplus	Retained Earnings	Retained Earnings Adoption of ASC 326	Retained Earnings Class A Common Stock	Retained Earnings Class B Common Stock	Accumulated Other Comprehensive (Loss) Income
Beginning balance, shareholders' equity 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	355,497								355,497
Other comprehensive income (loss), net of tax	63,415												63,415
Stock repurchased during period	(325,900)		$ (325,916)		(744)			(325,172)
Cash dividends			(12,262)	$ (1,206)							$ (12,262)	$ (1,206)
Preferred stock dividends declared				(9,426)								(9,426)
Ending balance, shareholders' equity at Sep. 30, 2019	3,568,482				9,879	1,005	0	168,790	3,560,580				(171,772)
Beginning balance, shareholders' equity at Jun. 30, 2019	3,574,613				10,175	1,005	0	303,880	3,440,284				(180,731)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	124,779								124,779
Other comprehensive income (loss), net of tax	8,959												8,959
Stock repurchased during period	(135,400)		(135,386)		(296)			(135,090)
Cash dividends			(4,081)	(402)							(4,081)	(402)
Ending balance, shareholders' equity at Sep. 30, 2019	3,568,482				9,879	1,005	0	168,790	3,560,580				(171,772)
Beginning balance, shareholders' equity at Dec. 31, 2019	3,586,184	$ 36,943			9,624	1,005	$ 0	$ 44,081	3,658,197	$ 36,943			(126,723)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	353,639								353,639
Other comprehensive income (loss), net of tax	$ 112,967												112,967
Issuance of preferred stock	339,937						339,937	0
Stock repurchased during period	$ (333,755)				(813)			$ (44,081)	(288,861)
Cash dividends			(10,869)	(1,206)							(10,869)	(1,206)
Ending balance, shareholders' equity at Sep. 30, 2020	4,074,414				8,811	1,005	$ 339,937	0	3,738,417				(13,756)
Beginning balance, shareholders' equity at Jun. 30, 2020	3,991,444				8,929	1,005	339,937	0	3,651,237				(9,664)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	142,684								142,684
Other comprehensive income (loss), net of tax	(4,092)												(4,092)
Stock repurchased during period	(47,060)				(118)			0	(46,942)
Cash dividends			$ (3,524)	(402)							$ (3,524)	(402)
Preferred stock dividends declared				$ (4,636)								$ (4,636)
Ending balance, shareholders' equity at Sep. 30, 2020	$ 4,074,414				$ 8,811	$ 1,005	$ 339,937	$ 0	$ 3,738,417				$ (13,756)