Consolidated Statements of Changes In Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock Class A Common Stock	Common Stock Class B Common Stock	Surplus	Retained Earnings [Member]	Accumulated Other Comprehensive Loss
Beginning balance at Dec. 31, 2014	$ 2,687,594	$ 11,005	$ 1,005	$ 658,918	$ 2,069,647	$ (52,981)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	210,386	0	0	0	210,386	0
Net current period other comprehensive income	(11,459)	0	0	0	0	(11,459)
Cash dividends	(14,412)	0	0	0	(14,412)	0
Ending 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)