Condensed Consolidated Statement of Comprehensive Income - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
		Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net earnings		$ 116,287	$ 115,735	$ 101,466	$ 350,298	$ 273,781
Other comprehensive (loss) income, net of tax:
Unrealized net holding gains (losses) on securities available-for-sale arising during the period		(29,267)	(14,325)	7,812	(106,261)	49,336
Income tax (expense) benefit related to unrealized holding gains (losses) arising during the period		8,344	4,102	(3,198)	30,377	(20,055)
Unrealized net holding gains (losses) on securities available-for-sale, net of tax		(20,923)	(10,223)	4,614	(75,884)	29,281
Reclassification adjustment for net (gains) losses included in net earnings	[1]	(826)	(253)	(1,236)	(7,390)	(2,788)
Income tax expense (benefit) related to reclassification adjustment		235	72	506	2,113	1,138
Reclassification adjustment for net (gains) losses included in net earnings, net of tax		(591)	(181)	(730)	(5,277)	(1,650)
Other comprehensive (loss) income, net of tax		(21,514)	(10,404)	3,884	(81,161)	27,631
Comprehensive income		$ 94,773	$ 105,331	$ 105,350	$ 269,137	$ 301,412

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