Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 158,873	$ 143,112	$ 94,482
Debt securities available-for-sale:
Net change in unrealized gains (losses) on debt securities available-for-sale	52,299	(9,228)	(1,702)
Related income tax (expense) benefit	(11,958)	2,008	717
Net securities (gains) losses reclassified into earnings	(227)	15	(42)
Related income tax expense (benefit)	52	(4)	15
Net effect on other comprehensive income for the period	40,166	(7,209)	(1,012)
Debt securities held-to-maturity:
Amortization of unrealized gain transferred from debt securities available-for-sale	(222)	(244)	(326)
Related income tax expense	54	56	117
Net effect on other comprehensive income for the period	(168)	(188)	(209)
Defined benefit plans:
Amortization of net loss and prior service costs	3,042	2,948	3,247
Related income tax benefit	(729)	(822)	(1,053)
Recognition of unrealized (loss) gain	(4,250)	(54)	380
Related income tax benefit (expense)	1,011	12	(141)
Net effect on other comprehensive income for the period	(926)	2,084	2,433
Total other comprehensive gain (loss)	39,072	(5,313)	1,212
Comprehensive income	$ 197,945	$ 137,799	$ 95,694