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	52,072	0	0	0	52,072	0
Other comprehensive income, net of tax	40,616	0	0	0	0	40,616
Cash dividends	(3,603)	0	0	0	(3,603)	0
Balance at Mar. 31, 2016	2,961,194	11,005	1,005	658,918	2,314,090	(23,824)
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	67,621	0	0	0	67,621	0
Other comprehensive income, net of tax	24,251	0	0	0	0	24,251
Cash dividends	(3,603)	0	0	0	(3,603)	0
Balance at Mar. 31, 2017	$ 3,100,696	$ 11,005	$ 1,005	$ 658,918	$ 2,540,709	$ (110,941)