Commitments and Contingencies (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating And Capital Leases Table [Abstract]
Operating Leases, Future Minimum Payments Due, Next Twelve Months	$ 5,378,000
Operating Leases, Future Minimum Payments, Due in Two Years	5,379,000
Operating Leases, Future Minimum Payments, Due in Three Years	6,945,000
Operating Leases, Future Minimum Payments, Due in Four Years	7,482,000
Operating Leases, Future Minimum Payments, Due in Five Years	7,629,000
Operating Leases, Future Minimum Payments, Due Thereafter	169,411,000
Capital Leases, Future Minimum Payments Due, Current	3,000
Capital Leases, Future Minimum Payments Due in Two Years	0
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	3,000
Future Minimum Payments Due Current	5,381,000
Future Minimum Payments Due In Two Years	5,379,000
Future Minimum Payments Due In Three Years	6,945,000
Future Minimum Payments Due In Four Years	7,482,000
Future Minimum Payments Due In Five Years	7,629,000
Future Minimum Payments Due Thereafter	169,411,000
Future Minimum Payments Due Total	202,227,000
Unrecorded Unconditional Purchase Obligations [Abstract]
Unrecorded Unconditional Purchase Obligation, Purchases	85,300,000	$ 85,000,000	$ 85,200,000
Unrecorded Unconditional Purchase Obligation Offsetting Sales	4,500,000	4,500,000	4,400,000
Unrecorded Unconditional Purchase Obligation [Line Items]
Operating Leases, Future Minimum Payments Due	$ 202,224,000
Leases [Abstract]
Land Buildings And Equipment Expiration Date	through 2108
Operating Leases, Rent Expense, Minimum Rentals	$ 7,500,000.0	$ 6,200,000.0	$ 5,500,000.0
Gas Purchase Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	63,944,000
Unrecorded Unconditional Purchase Obligation, Due within Two Years	2,729,000
Unrecorded Unconditional Purchase Obligation, Due within Three Years	2,729,000
Unrecorded Unconditional Purchase Obligation, Due within Four Years	2,273,000
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	71,675,000
Unrecorded Unconditional Purchase Obligation Amount Representing Interest	601,000
Unrecorded Unconditional Purchase Obligation Total At Present Value	71,074,000
Pipeline Capacity Purchase Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	79,891,000
Unrecorded Unconditional Purchase Obligation, Due within Two Years	82,129,000
Unrecorded Unconditional Purchase Obligation, Due within Three Years	77,028,000
Unrecorded Unconditional Purchase Obligation, Due within Four Years	65,630,000
Unrecorded Unconditional Purchase Obligation, Due within Five Years	60,050,000
Unrecorded Unconditional Purchase Obligation, Due after Five Years	601,844,000
Unrecorded Unconditional Purchase Obligation	966,572,000
Unrecorded Unconditional Purchase Obligation Amount Representing Interest	174,542,000
Unrecorded Unconditional Purchase Obligation Total At Present Value	792,030,000
Pipeline Capacity Release Agreements [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Unrecorded Unconditional Purchase Obligation, Due within One Year	3,581,000
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	3,581,000
Unrecorded Unconditional Purchase Obligation Amount Representing Interest	24,000
Unrecorded Unconditional Purchase Obligation Total At Present Value	$ 3,557,000
New Headquarters Lease 1st Renewal Period [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Lessee Leasing Arrangements, Operating Leases, Renewal Term	7 years
New Headquarters Lease 2nd Renewal Period [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Lessee Leasing Arrangements, Operating Leases, Renewal Term	7 years
New Headquarters Lease [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Lessee Leasing Arrangements, Operating Leases, Term of Contract	20 years
Lease Expected Commencement Year	1/1/2020
Operating Leases, Future Minimum Payments Due	$ 160,000,000
Current Headquarters Lease [Member]
Unrecorded Unconditional Purchase Obligation [Line Items]
Lease Expiration Date	Jan. 01, 2020