Consolidated Statements of Comprehensive Income - AOCI - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		$ 1,864,874	$ 1,257,080	$ 1,146,268
Other comprehensive (loss) income		(12,787)	7,874	(12,521)
Adjustments to apply recent accounting pronouncements	[1]		0
Ending balance		2,054,998	1,864,874	1,257,080
Accumulated Unrealized Loss on Securities Available- for-Sale
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		(13,976)	(22,645)	(10,271)
Other comprehensive (loss) income		(11,952)	8,669	(12,374)
Adjustments to apply recent accounting pronouncements	[1]	(2,864)
Ending balance		(28,792)	(13,976)	(22,645)
Accumulated Unrealized Loss on Derivative Instruments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		(3,763)	(1,176)	(2,468)
Other comprehensive (loss) income		1,997	(2,587)	1,292
Adjustments to apply recent accounting pronouncements	[1]	(784)
Ending balance		(2,550)	(3,763)	(1,176)
Unrecognized Net Pension Costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		(15,297)	(17,089)	(15,650)
Other comprehensive (loss) income		(2,832)	1,792	(1,439)
Adjustments to apply recent accounting pronouncements	[1]	(3,041)
Ending balance		(21,170)	(15,297)	(17,089)
Total Accumulated Other Comprehensive Loss, Net of Tax
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		(33,036)	(40,910)	(28,389)
Other comprehensive (loss) income		(12,787)	7,874	(12,521)
Adjustments to apply recent accounting pronouncements	[1]	(6,689)	(6,689)
Ending balance		$ (52,512)	$ (33,036)	$ (40,910)

[1] As a result of accounting guidance adopted in 2018, certain reclassifications were made from accumulated other comprehensive loss to retained earnings as of January 1, 2018. For further discussion of this guidance, see Note 2. "Recent Accounting Pronouncements."