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	Accumulated Other Comprehensive Loss
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	172,792	0	0	0	172,792	0
Other comprehensive income, net of tax	49,656	0	0	0	0	49,656
Cash dividends	(10,809)	0	0	0	(10,809)	0
Balance at Sep. 30, 2016	3,083,748	11,005	1,005	658,918	2,427,604	(14,784)
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	269,345	0	0	0		0
Other comprehensive income, net of tax	42,868	0	0	0	0	42,868
Cash dividends	(10,809)	0	0	0	(10,809)	0
Balance at Sep. 30, 2017	$ 3,313,831	$ 11,005	$ 1,005	$ 658,918	$ 2,735,227	$ (92,324)