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	$ 71,392	$ 67,081	$ 57,910
Other Comprehensive Income (Loss), Before Tax:
Net change in unrealized gains on securities available-for-sale	(2,899)	(663)	11,825
Net available-for-sale securities losses (gains) reclassified into earnings	0	34	(41)
Adjustment to funded status of employee benefit plans	6,974	(3,551)	(13,394)
Other Comprehensive Income (Loss), Before Tax	4,075	(4,180)	(1,610)
Income tax (expense) benefit related to items of other comprehensive income	(1,402)	1,556	471
Other Comprehensive Income (Loss), After Tax	2,673	(2,624)	(1,139)
Comprehensive Income	$ 74,065	$ 64,457	$ 56,771