Consolidated Statements of Changes In Shareholders' Equity - USD ($) $ in Thousands	Total	Class A Common Stock	Class B Common Stock	Common Stock Class A Common Stock	Common Stock Class B Common Stock	Surplus	Surplus Class A Common Stock	Retained Earnings	Retained Earnings Class A Common Stock	Retained Earnings Class B Common Stock	Accumulated Other Comprehensive Loss
Beginning balance at Dec. 31, 2016	$ 3,012,427			$ 11,005	$ 1,005	$ 658,918		$ 2,476,691			$ (135,192)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	323,752							323,752
Other comprehensive income (loss), net of tax	12,898										12,898
Cash dividends	(1,256)	$ (13,757)						(1,256)	$ (13,757)
Ending balance at Dec. 31, 2017	3,334,064			11,005	1,005	658,918		2,785,430			(122,294)
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	165,300	165,338		382			$ 164,956
Cash dividends		(15,785)	$ (1,458)	0	0	0			(15,785)	$ (1,458)	0
Ending balance at Dec. 31, 2018	3,488,954			10,623	1,005	493,962		3,218,551			(235,187)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	457,371							457,371			0
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)						$ (16,117)	$ (1,608)
Ending balance at Dec. 31, 2019	$ 3,586,184			$ 9,624	$ 1,005	$ 44,081		$ 3,658,197			$ (126,723)