Consolidated Statements of Comprehensive Income - AOCI - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		$ 2,054,998	$ 1,864,874
Adjustments to apply recent accounting pronouncements	[1]			$ 0
Ending balance		2,300,573	1,883,563
Accumulated Unrealized (Loss) Gain on Securities Available- for-Sale
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		(28,792)	(13,976)
Adjustments to apply recent accounting pronouncements	[2]			(2,864)
Other comprehensive income (loss)		46,889	(24,626)
Ending balance		18,097	(41,466)
Accumulated Unrealized (Loss) Gain on Derivative Instruments
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		(2,550)	(3,763)
Adjustments to apply recent accounting pronouncements	[2]			(784)
Other comprehensive income (loss)		2,813	(49)
Ending balance		263	(4,596)
Unrecognized Net Pension Costs
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		(21,170)	(15,297)
Adjustments to apply recent accounting pronouncements	[2]			(3,041)
Other comprehensive income (loss)		0	0
Ending balance		(21,170)	(18,338)
Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss) [Roll Forward]
Beginning balance		(52,512)	(33,036)
Adjustments to apply recent accounting pronouncements	[1],[2]			$ (6,689)
Other comprehensive income (loss)		49,702	(24,675)
Ending balance		$ (2,810)	$ (64,400)

[1] As a result of accounting guidance adopted in the first quarter of 2018, certain reclassifications were made from accumulated other comprehensive loss to retained earnings as of January 1, 2018.

[2] As a result of accounting guidance adopted in the first quarter of 2018, certain reclassifications were made from accumulated other comprehensive loss to retained earnings as of January 1, 2018.