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	121,388	0	0	0	121,388	0
Other comprehensive income, net of tax	49,413	0	0	0	0	49,413
Cash dividends	(7,206)	0	0	0	(7,206)	0
Balance at Jun. 30, 2016	3,035,704	11,005	1,005	658,918	2,379,803	(15,027)
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	202,281	0	0	0	202,281	0
Other comprehensive income, net of tax	32,349	0	0	0	0	32,349
Cash dividends	(7,206)	0	0	0	(7,206)	0
Balance at Jun. 30, 2017	$ 3,239,851	$ 11,005	$ 1,005	$ 658,918	$ 2,671,766	$ (102,843)