Commitments and Contingencies (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Leases [Abstract]
Land Buildings And Equipment Expiration Date	through 2108
Operating Leases, Rent Expense, Minimum Rentals	$ 5,500,000.0	$ 5,900,000.0	$ 5,100,000.0
Operating And Capital Leases Table [Abstract]
Operating Leases, Future Minimum Payments Due, Current	5,417,000
Operating Leases, Future Minimum Payments, Due in Two Years	5,363,000
Operating Leases, Future Minimum Payments, Due in Three Years	5,348,000
Operating Leases, Future Minimum Payments, Due in Four Years	5,313,000
Operating Leases, Future Minimum Payments, Due in Five Years	2,765,000
Operating Leases, Future Minimum Payments, Due Thereafter	30,475,000
Operating Leases, Future Minimum Payments Due	54,681,000
Capital Leases, Future Minimum Payments Due, Current	564,000
Capital Leases, Future Minimum Payments Due in Two Years	156,000
Capital Leases, Future Minimum Payments Due in Three Years	3,000
Capital Leases, Future Minimum Payments Due in Four Years	0
Capital Leases, Future Minimum Payments Due in Five Years	0
Capital Leases, Future Minimum Payments Due Thereafter	0
Capital Leases, Future Minimum Payments Due	723,000
Future Minimum Payments Due Current	5,981,000
Future Minimum Payments Due In Two Years	5,519,000
Future Minimum Payments Due In Three Years	5,351,000
Future Minimum Payments Due In Four Years	5,313,000
Future Minimum Payments Due In Five Years	2,765,000
Future Minimum Payments Due Thereafter	30,475,000
Future Minimum Payments Due Total	55,404,000
Unrecorded Unconditional Purchase Obligations [Abstract]
Unrecorded Unconditional Purchase Obligation, Purchases	85,200,000	94,300,000	98,200,000
Unrecorded Unconditional Purchase Obligation Offsetting Sales	4,400,000	$ 4,800,000	$ 4,500,000
Gas Purchase Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	61,464,000
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	61,464,000
Unrecorded Unconditional Purchase Obligation Amount Representing Interest	123,000
Unrecorded Unconditional Purchase Obligation Total At Present Value	61,341,000
Pipeline Capacity Purchase Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	79,487,000
Unrecorded Unconditional Purchase Obligation, Due within Two Years	79,370,000
Unrecorded Unconditional Purchase Obligation, Due within Three Years	75,796,000
Unrecorded Unconditional Purchase Obligation, Due within Four Years	75,683,000
Unrecorded Unconditional Purchase Obligation, Due within Five Years	72,091,000
Unrecorded Unconditional Purchase Obligation, Due after Five Years	340,027,000
Unrecorded Unconditional Purchase Obligation	722,454,000
Unrecorded Unconditional Purchase Obligation Amount Representing Interest	110,899,000
Unrecorded Unconditional Purchase Obligation Total At Present Value	611,555,000
Pipeline Capacity Release Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	3,739,000
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	3,739,000
Unrecorded Unconditional Purchase Obligation Amount Representing Interest	11,000
Unrecorded Unconditional Purchase Obligation Total At Present Value	$ 3,728,000