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
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	167,638	0	0	0	167,638	0
Net current period other comprehensive income	18,105	0	0	0	0	18,105
Cash dividends	(10,809)	0	0	0	(10,809)	0
Ending balance at Sep. 30, 2015	2,862,528	11,005	1,005	658,918	2,226,476	(34,876)
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	172,792	0	0	0	172,792	0
Net current period other comprehensive income	49,656	0	0	0	0	49,656
Cash dividends	(10,809)	0	0	0	(10,809)	0
Ending balance at Sep. 30, 2016	$ 3,083,748	$ 11,005	$ 1,005	$ 658,918	$ 2,427,604	$ (14,784)