Consolidated Statements of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 23,852	$ 20,484	$ 65,140	$ 59,469
Other comprehensive income (loss), net of tax
Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax	188	(816)	1,511	5,119
Net change in fair value of cash flow hedges	109	674	8	653
Net change in other comprehensive income for defined benefit postretirement plans	78	59	234	180
Total other comprehensive income (loss)	375	(83)	1,753	5,952
Total comprehensive income	$ 24,227	$ 20,401	$ 66,893	$ 65,421