Consolidated Statements of Comprehensive Income (not audited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement Of Income and Comprehensive Income [Abstract]
Net Income	$ 14,616	$ 15,392	$ 44,811	$ 44,763
Unrealized Gain on Available-for-Sale Securities:
Reversal of Previously Recognized Gains Realized on Sale of Investments and Included in Other Income During Period	(3)		(3)	(3)
(Losses) Gains Arising During Period	(35)	6	65	1
Income Tax Benefit (Expense)	13	(2)	(22)	1
Change in Unrealized Gains on Available-for-Sale Securities - net-of-tax	(25)	4	40	(1)
Pension and Postretirement Benefit Plans:
Amortization of Unrecognized Postretirement Benefit Losses and Costs (note 11)	161	205	470	616
Income Tax Expense	(64)	(82)	(188)	(247)
Pension and Postretirement Benefit Plans - net-of-tax	97	123	282	369
Total Other Comprehensive Income	72	127	322	368
Total Comprehensive Income	$ 14,688	$ 15,519	$ 45,133	$ 45,131