Condensed Consolidated Statement of Comprehensive Income - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net earnings		$ 115,735	$ 118,276	$ 93,647	$ 234,011	$ 172,315
Other comprehensive (loss) income, net of tax:
Unrealized net holding gains (losses) on securities available-for-sale arising during the period		(14,325)	(62,669)	30,340	(76,994)	41,524
Income tax (expense) benefit related to unrealized holding gains (losses) arising during the period		4,102	17,931	(12,350)	22,033	(16,857)
Unrealized net holding gains (losses) on securities available-for-sale, net of tax		(10,223)	(44,738)	17,990	(54,961)	24,667
Reclassification adjustment for net (gains) losses included in net earnings	[1]	(253)	(6,311)	(1,651)	(6,564)	(1,552)
Income tax expense (benefit) related to reclassification adjustment		72	1,806	672	1,878	632
Reclassification adjustment for net (gains) losses included in net earnings, net of tax		(181)	(4,505)	(979)	(4,686)	(920)
Other comprehensive (loss) income, net of tax		(10,404)	(49,243)	17,011	(59,647)	23,747
Comprehensive income		$ 105,331	$ 69,033	$ 110,658	$ 174,364	$ 196,062

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