Condensed Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net earnings		$ 33,204	$ (1,433,111)	$ 128,125	$ (1,399,907)	$ 240,729
Other comprehensive income, net of tax:
Unrealized net holding gains (losses) on securities available-for-sale arising during the period		82,780	17,183	72,168	99,963	134,807
Income tax (expense) benefit related to unrealized holding gains (losses) arising during the period		(23,095)	(4,794)	(20,459)	(27,889)	(38,217)
Unrealized net holding gains (losses) on securities available-for-sale, net of tax		59,685	12,389	51,709	72,074	96,590
Reclassification adjustment for net (gains) losses included in net earnings	[1]	(7,715)	(182)	(22,192)	(7,897)	(24,353)
Income tax expense (benefit) related to reclassification adjustment		2,152	51	6,291	2,203	6,904
Reclassification adjustment for net (gains) losses included in net earnings, net of tax		(5,563)	(131)	(15,901)	(5,694)	(17,449)
Other comprehensive income, net of tax		54,122	12,258	35,808	66,380	79,141
Comprehensive income (loss)		$ 87,326	$ (1,420,853)	$ 163,933	$ (1,333,527)	$ 319,870

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