Condensed Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands		Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Treasury Stock	Shares Of Common Stock	Accumulated Other Comprehensive Income (Loss)
Dividends declared per share (usd per share)		$ 0.50
Beginning Period Balance at Dec. 31, 2016		$ 4,479,055	$ 1,228	$ 4,162,132	$ 366,073	$ (56,360)		$ 5,982
Shares outstanding, beginning balance (shares) at Dec. 31, 2016			121,283,669
Stockholders' Equity Disclosure [Rollforward]
Cumulative effect of change in accounting principle	[1]	291		711	(420)
Net earnings		78,668			78,668
Other comprehensive income - net unrealized gain on securities available-for-sale, net of tax		6,736						6,736
Restricted stock awarded and earned compensation, net of shares forfeited (shares)			165,648
Restricted stock awarded and earned stock compensation, net of shares forfeited		6,470	$ 2	6,468
Restricted stock surrendered (shares)			(41,184)
Restricted stock surrendered		(2,281)				(2,281)
Cash dividends paid		(60,833)		(60,833)
Ending Period Balance at Mar. 31, 2017		4,508,106	$ 1,230	4,108,478	444,321	(58,641)		12,718
Shares outstanding, ending balance (shares) at Mar. 31, 2017			121,408,133
Beginning Period Balance at Dec. 31, 2016		4,479,055	$ 1,228	4,162,132	366,073	(56,360)		5,982
Shares outstanding, beginning balance (shares) at Dec. 31, 2016			121,283,669
Stockholders' Equity Disclosure [Rollforward]
Net earnings		273,781
Ending Period Balance at Sep. 30, 2017		$ 4,610,668	$ 1,231	4,000,543	639,434	(64,153)		33,613
Shares outstanding, ending balance (shares) at Sep. 30, 2017			121,449,794
Dividends declared per share (usd per share)		$ 0.50
Beginning Period Balance at Mar. 31, 2017		$ 4,508,106	$ 1,230	4,108,478	444,321	(58,641)		12,718
Shares outstanding, beginning balance (shares) at Mar. 31, 2017			121,408,133
Stockholders' Equity Disclosure [Rollforward]
Net earnings		93,647			93,647
Other comprehensive income - net unrealized gain on securities available-for-sale, net of tax		17,011						17,011
Restricted stock awarded and earned compensation, net of shares forfeited (shares)			147,850
Restricted stock awarded and earned stock compensation, net of shares forfeited		7,249	$ 1	7,248
Restricted stock surrendered (shares)			(107,662)
Restricted stock surrendered		(5,277)				(5,277)
Cash dividends paid		(60,831)		(60,831)
Ending Period Balance at Jun. 30, 2017		$ 4,559,905	$ 1,231	4,054,895	537,968	(63,918)		29,729
Shares outstanding, ending balance (shares) at Jun. 30, 2017			121,448,321
Dividends declared per share (usd per share)		$ 0.50
Stockholders' Equity Disclosure [Rollforward]
Net earnings		$ 101,466			101,466
Other comprehensive income - net unrealized gain on securities available-for-sale, net of tax		3,884						3,884
Restricted stock awarded and earned compensation, net of shares forfeited (shares)			6,365
Restricted stock awarded and earned stock compensation, net of shares forfeited		6,479	$ 0	6,479
Restricted stock surrendered (shares)			(4,892)
Restricted stock surrendered		(235)				(235)
Cash dividends paid		(60,831)		(60,831)
Ending Period Balance at Sep. 30, 2017		$ 4,610,668	$ 1,231	4,000,543	639,434	(64,153)		33,613
Shares outstanding, ending balance (shares) at Sep. 30, 2017			121,449,794
Dividends declared per share (usd per share)		$ 0.50
Beginning Period Balance at Dec. 31, 2017		$ 4,977,598	$ 1,305	4,287,487	723,471	(65,836)		31,171
Shares outstanding, beginning balance (shares) at Dec. 31, 2017			128,782,878
Stockholders' Equity Disclosure [Rollforward]
Cumulative effect of change in accounting principle	[1]	0			(6,136)			6,136
Net earnings		118,276			118,276
Other comprehensive income - net unrealized gain on securities available-for-sale, net of tax		(49,243)						(49,243)
Restricted stock awarded and earned compensation, net of shares forfeited (shares)			96,034
Restricted stock awarded and earned stock compensation, net of shares forfeited		7,199	$ 1	7,198
Restricted stock surrendered (shares)			(55,186)				(2,285,855)
Restricted stock surrendered		(2,858)				(2,858)
Common stock repurchased under Stock Repurchase Program		(23)
Common stock repurchased under Stock Repurchase Program		(119,793)		(119,770)		0
Cash dividends paid		(63,689)		(63,689)
Ending Period Balance at Mar. 31, 2018		4,867,490	$ 1,283	4,111,226	835,611	(68,694)		(11,936)
Shares outstanding, ending balance (shares) at Mar. 31, 2018			126,537,871
Beginning Period Balance at Dec. 31, 2017		4,977,598	$ 1,305	4,287,487	723,471	(65,836)		31,171
Shares outstanding, beginning balance (shares) at Dec. 31, 2017			128,782,878
Stockholders' Equity Disclosure [Rollforward]
Net earnings		350,298
Ending Period Balance at Sep. 30, 2018		$ 4,741,685	$ 1,251	3,789,893	1,067,633	(73,238)		(43,854)
Shares outstanding, ending balance (shares) at Sep. 30, 2018			123,283,450
Dividends declared per share (usd per share)		$ 0.60
Beginning Period Balance at Mar. 31, 2018		$ 4,867,490	$ 1,283	4,111,226	835,611	(68,694)		(11,936)
Shares outstanding, beginning balance (shares) at Mar. 31, 2018			126,537,871
Stockholders' Equity Disclosure [Rollforward]
Net earnings		115,735			115,735
Other comprehensive income - net unrealized gain on securities available-for-sale, net of tax		(10,404)						(10,404)
Restricted stock awarded and earned compensation, net of shares forfeited (shares)			398,132
Restricted stock awarded and earned stock compensation, net of shares forfeited		7,546	$ 4	7,542
Restricted stock surrendered (shares)			(81,172)				(2,286,881)
Restricted stock surrendered		(4,332)				(4,332)
Common stock repurchased under Stock Repurchase Program		(23)
Common stock repurchased under Stock Repurchase Program		(122,024)		(122,001)		0
Cash dividends paid		(76,052)		(76,052)
Ending Period Balance at Jun. 30, 2018		$ 4,777,959	$ 1,264	3,920,715	951,346	(73,026)		(22,340)
Shares outstanding, ending balance (shares) at Jun. 30, 2018			124,567,950
Dividends declared per share (usd per share)		$ 0.60
Stockholders' Equity Disclosure [Rollforward]
Net earnings		$ 116,287			116,287
Other comprehensive income - net unrealized gain on securities available-for-sale, net of tax		(21,514)						(21,514)
Restricted stock awarded and earned compensation, net of shares forfeited (shares)			(3,803)
Restricted stock awarded and earned stock compensation, net of shares forfeited		8,137	$ 0	8,137
Restricted stock surrendered (shares)			(4,199)				(1,276,498)
Restricted stock surrendered		(212)				(212)
Common stock repurchased under Stock Repurchase Program		(13)
Common stock repurchased under Stock Repurchase Program		(64,577)		(64,564)		0
Cash dividends paid		(74,395)		(74,395)
Ending Period Balance at Sep. 30, 2018		$ 4,741,685	$ 1,251	$ 3,789,893	$ 1,067,633	$ (73,238)		$ (43,854)
Shares outstanding, ending balance (shares) at Sep. 30, 2018			123,283,450

[1] Impact due to adoption on January 1, 2018 of ASU 2016-01, "Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities" and ASU 2018-02, "Income Statement - Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income."