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, 2012	$ 1,859,624				$ 8,588	$ 1,033	$ 143,766			$ 1,788,343			$ (82,106)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	166,869				0	0	0			166,869			0
Net current period other comprehensive income	56,838				0	0	0			0			56,838
Repurchase of common stock shares		$ (321)			(2)	0		$ 0			$ (319)			$ 0
Repurchase and retirement of common stock	0
Cash dividends	(11,548)				0	0	0			(11,548)			0
Ending 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)