Condensed Consolidated Statement of Comprehensive Income - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Net earnings		$ 93,647	$ 78,668	$ 82,168	$ 172,315	$ 172,624
Other comprehensive income (loss), net of tax:
Unrealized net holding gains (losses) on securities available-for-sale arising during the period		30,340	11,184	56,514	41,524	99,607
Income tax (expense) benefit related to unrealized holding gains (losses) arising during the period		(12,350)	(4,507)	(22,965)	(16,857)	(40,620)
Unrealized net holding gains (losses) on securities available-for-sale, net of tax		17,990	6,677	33,549	24,667	58,987
Reclassification adjustment for net (gains) losses included in net earnings	[1]	(1,651)	99	(478)	(1,552)	(8,588)
Income tax expense (benefit) related to reclassification adjustment		672	(40)	194	632	3,517
Reclassification adjustment for net (gains) losses included in net earnings, net of tax		(979)	59	(284)	(920)	(5,071)
Other comprehensive income, net of tax		17,011	6,736	33,265	23,747	53,916
Comprehensive income		$ 110,658	$ 85,404	$ 115,433	$ 196,062	$ 226,540

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