Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net Income	$ 184,672	$ 181,461	$ 160,704
Other Comprehensive Income (Loss), Net of Tax:
Net Unrealized Gains (Losses) on Investment Securities	(1,986)	(10,318)	(2,125)
Defined Benefit Plans, Net	1,177	(31)	5,254
Other Comprehensive Income (Loss)	(809)	(10,349)	3,129
Comprehensive Income	$ 183,863	$ 171,112	$ 163,833