Consolidated Statements of Changes In Shareholders' Equity - USD ($) $ in Thousands	Total	Class A Common Stock	Class B Common Stock	Common Stock Class A Common Stock	Common Stock Class B Common Stock	Surplus	Surplus Class A Common Stock	Retained Earnings	Retained Earnings Class A Common Stock	Retained Earnings Class B Common Stock	Accumulated Other Comprehensive (Loss) Income	Financial Instruments ASU cumulative effect adjustment Retained Earnings	Financial Instruments ASU cumulative effect adjustment Accumulated Other Comprehensive (Loss) Income	Income tax ASU cumulative effect adjustment Retained Earnings	Income tax ASU cumulative effect adjustment Accumulated Other Comprehensive (Loss) Income
Beginning balance, shareholders' equity at Dec. 31, 2017	$ 3,334,064			$ 11,005	$ 1,005	$ 658,918		$ 2,785,430			$ (122,294)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	310,840							310,840
Other comprehensive income (loss), net of tax	(75,230)										(75,230)
Stock repurchased during period	(58,086)			(125)			$ (57,961)
Cash dividends		$ (11,520)	$ (1,055)						$ (11,520)	$ (1,055)
Ending balance, shareholders' equity at Sep. 30, 2018	3,499,013			10,880	1,005	600,957		3,133,746			(247,575)
Beginning balance, shareholders' equity at Jun. 30, 2018	3,446,886			11,005	1,005	658,918		3,020,596			(244,638)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	117,318							117,318
Other comprehensive income (loss), net of tax	(2,937)										(2,937)
Stock repurchased during period	(58,086)			(125)			(57,961)	0
Cash dividends		(3,817)	(351)						(3,817)	(351)
Ending balance, shareholders' equity at Sep. 30, 2018	3,499,013			10,880	1,005	600,957		3,133,746			(247,575)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of new accounting principle in period of adoption												$ 18,715	$ (18,715)	$ 31,336	$ (31,336)
Beginning balance, shareholders' equity at Dec. 31, 2018	3,488,954			10,623	1,005	493,962		3,218,551			(235,187)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	355,497
Other comprehensive income (loss), net of tax	63,415										63,415
Stock repurchased during period	(325,900)	(325,916)		(744)			(325,172)
Cash dividends		(12,262)	(1,206)						(12,262)	(1,206)
Ending balance, shareholders' equity at Sep. 30, 2019	3,568,482			9,879	1,005	168,790		3,560,580			(171,772)
Beginning balance, shareholders' equity at Jun. 30, 2019	3,574,613			10,175	1,005	303,880		3,440,284			(180,731)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	124,779							124,779
Other comprehensive income (loss), net of tax	8,959										8,959
Stock repurchased during period	(135,400)	(135,386)		(296)			$ (135,090)
Cash dividends		$ (4,081)	$ (402)						$ (4,081)	$ (402)
Ending balance, shareholders' equity at Sep. 30, 2019	$ 3,568,482			$ 9,879	$ 1,005	$ 168,790		$ 3,560,580			$ (171,772)