Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 86,635	$ 80,762	$ 69,974
Securities available-for-sale:
Net change in unrealized (losses) gains on securities available-for-sale	(6,761)	(10,552)	15,242
Related income tax benefit (expense)	2,461	3,875	(5,604)
Net securities gains reclassified into earnings	(2,251)	(596)	(981)
Related income tax expense	823	219	361
Net effect on other comprehensive income for the period	(5,728)	(7,054)	9,018
Securities held-to-maturity:
Amortization of unrealized gain transferred from available-for-sale	(357)	(494)	(472)
Related income tax expense	132	182	173
Net effect on other comprehensive income for the period	(225)	(312)	(299)
Defined benefit pension plan:
Amortization of net loss and prior service costs	3,046	3,205	1,516
Related income tax benefit	(1,153)	(1,201)	(558)
Recognition of unrealized (loss) gain	(3,329)	5,106	(24,934)
Related income tax benefit (expense)	1,217	(1,873)	9,166
Net effect on other comprehensive income for the period	(219)	5,237	(14,810)
Total other comprehensive loss	(6,172)	(2,129)	(6,091)
Comprehensive income	$ 80,463	$ 78,633	$ 63,883