Transactions with Affiliates (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Amounts paid to the Williams Entities:
Percentage of ownership of affiliates of the company in the partnership	31.90%
Percentage of owner of company in affiliated partnership	25.80%
Affiliated Entity
Amounts received from the Williams Entities:
Services	$ 663	$ 715	$ 671
Insurance premiums and benefits	960	837	920
Reimbursed expenses	296	427	566
Amount received from Williams Entities	1,919	1,979	2,157
Amounts paid to the Williams Entities:
Rent	1,614	1,560	1,478
Business entertainment	205	344	116
Reimbursed expenses	204	216	267
Amounts paid to the Williams Entities	$ 2,023	$ 2,120	$ 1,861