Commitments and Contingencies (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Leases [Abstract]
Land Buildings And Equipment Expiration Date	through 2108
Operating Leases, Rent Expense, Minimum Rentals	$ 6,200,000.0	$ 5,500,000.0	$ 5,900,000.0
Operating And Capital Leases Table [Abstract]
Operating Leases, Future Minimum Payments Due, Current	5,476,000
Operating Leases, Future Minimum Payments, Due in Two Years	5,385,000
Operating Leases, Future Minimum Payments, Due in Three Years	5,340,000
Operating Leases, Future Minimum Payments, Due in Four Years	2,835,000
Operating Leases, Future Minimum Payments, Due in Five Years	930,000
Operating Leases, Future Minimum Payments, Due Thereafter	28,895,000
Operating Leases, Future Minimum Payments Due	48,861,000
Capital Leases, Future Minimum Payments Due, Current	156,000
Capital Leases, Future Minimum Payments Due in Two Years	3,000
Capital Leases, Future Minimum Payments Due in Three Years	0
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	159,000
Future Minimum Payments Due Current	5,632,000
Future Minimum Payments Due In Two Years	5,388,000
Future Minimum Payments Due In Three Years	5,340,000
Future Minimum Payments Due In Four Years	2,835,000
Future Minimum Payments Due In Five Years	930,000
Future Minimum Payments Due Thereafter	28,895,000
Future Minimum Payments Due Total	49,020,000
Unrecorded Unconditional Purchase Obligations [Abstract]
Unrecorded Unconditional Purchase Obligation, Purchases	85,000,000	85,200,000	94,300,000
Unrecorded Unconditional Purchase Obligation Offsetting Sales	4,500,000	$ 4,400,000	$ 4,800,000
Gas Purchase Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	78,587,000
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	78,587,000
Unrecorded Unconditional Purchase Obligation Amount Representing Interest	220,000
Unrecorded Unconditional Purchase Obligation Total At Present Value	78,367,000
Pipeline Capacity Purchase Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	81,206,000
Unrecorded Unconditional Purchase Obligation, Due within Two Years	79,741,000
Unrecorded Unconditional Purchase Obligation, Due within Three Years	77,125,000
Unrecorded Unconditional Purchase Obligation, Due within Four Years	72,021,000
Unrecorded Unconditional Purchase Obligation, Due within Five Years	45,971,000
Unrecorded Unconditional Purchase Obligation, Due after Five Years	296,592,000
Unrecorded Unconditional Purchase Obligation	652,656,000
Unrecorded Unconditional Purchase Obligation Amount Representing Interest	101,576,000
Unrecorded Unconditional Purchase Obligation Total At Present Value	551,080,000
Pipeline Capacity Release Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	4,487,000
Unrecorded Unconditional Purchase Obligation, Due within Two Years	3,739,000
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	8,226,000
Unrecorded Unconditional Purchase Obligation Amount Representing Interest	94,000
Unrecorded Unconditional Purchase Obligation Total At Present Value	$ 8,132,000