Consolidated Statements of Changes In Shareholders' Equity - USD ($) $ in Thousands	Total	Class A Common Stock	Class B Common Stock	Common Stock	Common Stock Class A Common Stock	Common Stock Class B Common Stock	Surplus	Surplus Class A Common Stock	Surplus Class B Common Stock	Retained Earnings [Member]	Retained Earnings [Member] Class A Common Stock	Retained Earnings [Member] Class B Common Stock	Accumulated Other Comprehensive Loss	Accumulated Other Comprehensive Loss Class A Common Stock	Accumulated Other Comprehensive Loss Class B Common Stock
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	138,562				0	0	0			138,562			0
Net current period other comprehensive income	(27,713)				0	0	0			0			(27,713)
Stock Issued During Period, Value, New Issues	563,628				2,587	18	561,023			0			0
Repurchase of common stock shares		$ (36,308)	$ (9,777)	$ 0	(168)	(46)		$ (36,140)	$ (9,731)		$ 0	$ 0		$ 0	$ 0
Repurchase and retirement of common stock	46,085
Cash dividends	(12,260)				0	0	0			(12,260)			0
Ending 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	210,386				0	0	0			210,386			0
Net current period other comprehensive income	(11,459)				0	0	0			0			(11,459)
Repurchase and retirement of common stock	0
Cash dividends	(14,412)				0	0	0			(14,412)			0
Ending 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	225,482				0	0	0			225,482			0
Net current period other comprehensive income	(70,752)				0	0	0			0			(70,752)
Repurchase and retirement of common stock	0
Cash dividends	(14,412)				0	0	0			(14,412)			0
Ending balance at Dec. 31, 2016	$ 3,012,427				$ 11,005	$ 1,005	$ 658,918			$ 2,476,691			$ (135,192)