Commitments and Contingencies (details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Leases [Abstract]
Land Buildings And Equipment Expiration Date	various years through 2108
Operating Leases, Rent Expense, Minimum Rentals	$ 4,800,000	$ 5,400,000	$ 5,100,000
Operating And Capital Leases Table [Abstract]
Operating Leases, Future Minimum Payments Due, Current	5,415,000
Operating Leases, Future Minimum Payments, Due in Two Years	5,655,000
Operating Leases, Future Minimum Payments, Due in Three Years	5,498,000
Operating Leases, Future Minimum Payments, Due in Four Years	5,478,000
Operating Leases, Future Minimum Payments, Due in Five Years	5,474,000
Operating Leases, Future Minimum Payments, Due Thereafter	33,187,000
Operating Leases, Future Minimum Payments Due	60,707,000
Capital Leases, Future Minimum Payments Due, Current	547,000
Capital Leases, Future Minimum Payments Due in Two Years	335,000
Capital Leases, Future Minimum Payments Due in Three Years	136,000
Capital Leases, Future Minimum Payments Due in Four Years	42,000
Capital Leases, Future Minimum Payments Due in Five Years	1,000
Capital Leases, Future Minimum Payments Due Thereafter	0
Capital Leases, Future Minimum Payments Due	1,061,000
Future Minimum Payments Due Current	5,962,000
Future Minimum Payments Due In Two Years	5,990,000
Future Minimum Payments Due In Three Years	5,634,000
Future Minimum Payments Due In Four Years	5,520,000
Future Minimum Payments Due In Five Years	5,475,000
Future Minimum Payments Due Thereafter	33,187,000
Future Minimum Payments Due Total	61,768,000
Unrecorded Unconditional Purchase Obligations [Abstract]
Unrecorded Unconditional Purchase Obligation, Purchases	94,300,000	94,200,000	91,400,000
Unrecorded Unconditional Purchase Obligation Offsetting Sales	4,200,000	3,100,000	4,200,000
Gas Purchase Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	104,443,000
Unrecorded Unconditional Purchase Obligation, Due within Two Years	12,166,000
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	116,609,000
Unrecorded Unconditional Purchase Obligation Amount Representing Interest	129,000
Unrecorded Unconditional Purchase Obligation Total At Present Value	116,480,000
Pipeline Capacity Purchase Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	90,823,000
Unrecorded Unconditional Purchase Obligation, Due within Two Years	86,119,000
Unrecorded Unconditional Purchase Obligation, Due within Three Years	72,707,000
Unrecorded Unconditional Purchase Obligation, Due within Four Years	61,398,000
Unrecorded Unconditional Purchase Obligation, Due within Five Years	48,503,000
Unrecorded Unconditional Purchase Obligation, Due after Five Years	240,929,000
Unrecorded Unconditional Purchase Obligation	600,479,000
Unrecorded Unconditional Purchase Obligation Amount Representing Interest	87,263,000
Unrecorded Unconditional Purchase Obligation Total At Present Value	513,216,000
Pipeline Capacity Release Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	3,464,000
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	3,464,000
Unrecorded Unconditional Purchase Obligation Amount Representing Interest	0
Unrecorded Unconditional Purchase Obligation Total At Present Value	$ 3,464,000