Consolidated Statements of Changes In Shareholders' Equity - USD ($) $ in Thousands	Total	Class A Common Stock	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	Accumulated Other Comprehensive Loss	Accumulated Other Comprehensive Loss Class A Common Stock
Beginning balance at Dec. 31, 2015	$ 2,872,109			$ 11,005	$ 1,005	$ 658,918		$ 2,265,621		$ (64,440)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	225,482			0	0	0		225,482		0
Net current period other comprehensive income	(70,752)			0	0	0		0		(70,752)
Cash dividends	(14,412)			0	0	0		(14,412)		0
Ending 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			0	0	0		323,752		0
Net current period other comprehensive income	12,898			0	0	0		0		12,898
Cash dividends	(15,013)			0	0	0		(15,013)		0
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			0	0	0		400,313		0
Net current period other comprehensive income	(62,842)			0	0	0		0		(62,842)
Repurchase and retirement of common stock		$ 165,338	$ 0	382			$ 164,956		$ 0		$ 0
Cash dividends	(17,243)			0	0	0		(17,243)		0
Ending balance at Dec. 31, 2018	$ 3,488,954			$ 10,623	$ 1,005	$ 493,962		$ 3,218,551		$ (235,187)