Other-Than-Temporary Impairment Roll-forward of Amounts Related to Credit Losses Recognized in Earnings (Details) - Held-to-maturity Securities [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance at beginning of year	$ 422,035	$ 452,523	$ 490,404
Credit losses for which other-than-temporary impairment was not previously recognized	128	0	0
Additional credit losses for which an other-than-temporary impairment charge was previously recognized	1,084	532	1,454
Securities sold or matured during the year	(56,710)	0	(5,600)
Portion of increase in cash flows expected to be collected over the remaining life of the security that are recognized in the current period as interest income	(26,907)	(31,020)	(33,735)
Balance at end of year	$ 339,630	$ 422,035	$ 452,523