Non-Qualified Retirement, Savings and Investment Plans (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015 USD ($) plan	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) plan	Jun. 30, 2014 USD ($)	Dec. 31, 2009	Dec. 31, 2014 USD ($)
Non-Qualified Retirement, Savings and Investment Plans [Line Items]
Number of non-qualified retirement savings and investment plans | plan	2		2
Assets held in trust, current	$ 174		$ 174			$ 214
Investments held in trust	18,274		18,274			17,539
Executive Deferred Compensation Plan [Member]
Non-Qualified Retirement, Savings and Investment Plans [Line Items]
Investment yield, basis spread on variable rate					3.00%
Deferred compensation liability, current and long-term	10,400		10,400			10,200
Compensation expense	100	$ 200	300	$ 400
Diversified investments held in trust	5,300		5,300			4,600
Assets held in trust, current	200		200
Investment gains (losses)	(42)	100	100	100
Common stock held by plan	200		200			200
Non-Qualified Plan [Member]
Non-Qualified Retirement, Savings and Investment Plans [Line Items]
Deferred compensation liability, current and long-term	14,000		14,000			14,400
Investment gains (losses)	(100)	300	(11)	400
Common stock held by plan	900		900			1,300
Increase (decrease) in compensation expense	(400)	$ 400	(200)	$ 200
Investments held in trust	$ 13,100		$ 13,100			$ 13,100