CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Net income	$ 102,273	$ 167,377	$ 201,608
Debt securities:
Unrealized gain on debt securities for which credit losses has been recognized	772	48	255
Reclassification adjustment for provision for credit loss expense on available for sale debt securities	1,641	497	50
Reclassification adjustment for net gain included in net income on sales of available-for-sale securities with no credit losses previously recognized	(13,198)	0	34
All other unrealized holding (losses) gains on available-for-sale securities	59,746	46,634	(20,145)
Defined benefit plans adjustments:
Net actuarial loss	(270)	0	0
Other comprehensive (loss) income for the year	48,691	47,179	(19,806)
Total comprehensive income	150,964	214,556	181,802
Other Comprehensive Income (Loss), Tax [Abstract]
Unrealized gain on debt securities for which credit losses have been recognized, tax	0	0	0
Reclassification adjustment for credit losses on debt securities included in net income, tax	0	0	0
Reclassification adjustments for net (gain) loss included in net income on sales of available-for-sale debt securities with no credit losses previously recognized, tax	0	0	0
All other unrealized holding gains (losses) on available-for-sale debt securities, tax	0	0	0
Net actuarial loss, tax	(162)	0	0
Total	$ (162)	$ 0	$ 0