CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Net income	$ 6,667	$ 7,265
Other comprehensive income (loss), net of tax:
Total other comprehensive income, net	1,367	1,585
Comprehensive income	8,034	8,850
Defined Benefit Pension Plans [Member]
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss), net of tax	2,681	2,868
OPEB [Member]
Other comprehensive income (loss), net of tax:
Other comprehensive income (loss), net of tax	$ (1,314)	$ (1,283)