Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 122,044	$ 158,873	$ 143,112
Debt securities available-for-sale:
Net change in unrealized gains (losses) on debt securities available-for-sale	39,880	52,299	(9,228)
Related income tax (expense) benefit	(9,727)	(11,958)	2,008
Net securities (gains) losses reclassified into earnings	(2,540)	(227)	15
Related income tax expense (benefit)	604	52	(4)
Net effect on other comprehensive income for the period	28,217	40,166	(7,209)
Debt securities held-to-maturity:
Amortization of unrealized gain transferred from debt securities available-for-sale	(32)	(222)	(244)
Related income tax expense	7	54	56
Net effect on other comprehensive income for the period	(25)	(168)	(188)
Defined benefit plans:
Amortization of net loss and prior service costs	3,000	3,042	2,948
Related income tax benefit	(714)	(729)	(822)
Recognition of unrealized loss	(420)	(4,250)	(54)
Related income tax benefit	100	1,011	12
Net effect on other comprehensive income for the period	1,966	(926)	2,084
Total other comprehensive gain (loss)	30,158	39,072	(5,313)
Comprehensive income	$ 152,202	$ 197,945	$ 137,799