Condensed Consolidated Statement of Comprehensive Income - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
		Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net earnings		$ 101,466	$ 93,647	$ 93,895	$ 273,781	$ 266,519
Other comprehensive income (loss), net of tax:
Unrealized net holding gains (losses) on securities available-for-sale arising during the period		7,812	30,340	(15,954)	49,336	83,653
Income tax (expense) benefit related to unrealized holding gains (losses) arising during the period		(3,198)	(12,350)	6,509	(20,055)	(34,111)
Unrealized net holding gains (losses) on securities available-for-sale, net of tax		4,614	17,990	(9,445)	29,281	49,542
Reclassification adjustment for net (gains) losses included in net earnings	[1]	(1,236)	(1,651)	(382)	(2,788)	(8,970)
Income tax expense (benefit) related to reclassification adjustment		506	672	156	1,138	3,673
Reclassification adjustment for net (gains) losses included in net earnings, net of tax		(730)	(979)	(226)	(1,650)	(5,297)
Other comprehensive income (loss), net of tax		3,884	17,011	(9,671)	27,631	44,245
Comprehensive income		$ 105,350	$ 110,658	$ 84,224	$ 301,412	$ 310,764

[1] Entire amounts are recognized in "Gain on sale of securities" on the Condensed Consolidated Statements of Earnings.