Consolidated Statement of Comprehensive Income - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net earnings		$ 357,818	$ 352,166	$ 299,619
arising during the year		42,190	(27,392)	6,490
arising during the year		(17,481)	11,148	(2,869)
Unrealized net holding gains (losses) on securities available-for-sale, net of tax		24,709	(16,244)	3,621
Reclassification adjustment for net losses (gains) included in net earnings (1)	[1]	541	(9,485)	(3,744)
Income tax (benefit) expense related to reclassification adjustment		(61)	3,883	1,571
net of tax		480	(5,602)	(2,173)
Other comprehensive income (loss), net of tax		25,189	(21,846)	1,448
Comprehensive income		$ 383,007	$ 330,320	$ 301,067

[1] Entire amount recognized in "(Loss) gain on sale of securities" on the Consolidated Statements of Earnings.