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	67,158	0	0	0	67,158	0
Net current period other comprehensive income	17,570	0	0	0	0	17,570
Cash dividends	(3,603)	0	0	0	(3,603)	0
Ending balance at Mar. 31, 2015	2,768,719	11,005	1,005	658,918	2,133,202	(35,411)
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	52,072	0	0	0		0
Net current period other comprehensive income	40,616	0	0	0	0	40,616
Cash dividends	(3,603)	0	0	0	(3,603)	0
Ending balance at Mar. 31, 2016	$ 2,961,194	$ 11,005	$ 1,005	$ 658,918	$ 2,314,090	$ (23,824)