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, 2013	$ 2,071,462	$ 8,586	$ 1,033	$ 143,766	$ 1,943,345	$ (25,268)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	75,666	0	0	0	75,666	0
Net current period other comprehensive income	23,410	0	0	0	0	23,410
Cash dividends	(8,657)	0	0	0	(8,657)	0
Ending balance at Sep. 30, 2014	2,161,881	8,586	1,033	143,766	2,010,354	(1,858)
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		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)