Fair value measurements (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012		Dec. 31, 2011		Jun. 30, 2011
Concentration risks, percentage [Abstract]
Percentage investment in common stock of mid-cap companies	28.00%		33.00%		34.00%
Percentage investment in common stock of small-cap companies	28.00%		34.00%		33.00%
Percentage investment in common stock of large-cap companies	29.00%		32.00%		32.00%
Percentage in fixed-income and other investments	15.00%
Percentage investment in cash and cash equivalents			1.00%		1.00%
Fair value, measurements, recurring [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	125,917		187,659		90,655
Liabilities, fair value disclosure	8,000		18,863		25,421
Fair value, measurements, recurring [Member] | Commodity derivative instruments, liabilities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Liabilities, fair value disclosure	1,037		14,101		25,421
Fair value, measurements, recurring [Member] | Interest rate swap [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Liabilities, fair value disclosure	6,963		4,762
Fair value, measurements, recurring [Member] | Money market funds [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	21,054		97,500		8,297
Fair value, measurements, recurring [Member] | Insurance investment contract [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	45,959	[1]	38,352	[2]	40,328	[3]
Fair value, measurements, recurring [Member] | Auction rate securities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure			11,400		11,400
Fair value, measurements, recurring [Member] | Collateralized mortgage backed securities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	8,253		8,296		8,162
Fair value, measurements, recurring [Member] | US Treasury securities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	1,994		1,656		1,969
Fair value, measurements, recurring [Member] | Commodity derivative instruments , assets [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	48,657		30,455		20,499
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	0		0		0
Liabilities, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Commodity derivative instruments, liabilities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Liabilities, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Interest rate swap [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Liabilities, fair value disclosure	0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Money market funds [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Insurance investment contract [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	0	[1]	0	[2]	0	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Auction rate securities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure			0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Collateralized mortgage backed securities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | US Treasury securities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 [Member] | Commodity derivative instruments , assets [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	125,917		187,659		90,655
Liabilities, fair value disclosure	8,000		18,863		25,421
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments, liabilities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Liabilities, fair value disclosure	1,037		14,101		25,421
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Interest rate swap [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Liabilities, fair value disclosure	6,963		4,762
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Money market funds [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	21,054		97,500		8,297
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Insurance investment contract [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	45,959	[1]	38,352	[2]	40,328	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Auction rate securities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure			11,400		11,400
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Collateralized mortgage backed securities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	8,253		8,296		8,162
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | US Treasury securities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	1,994		1,656		1,969
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 [Member] | Commodity derivative instruments , assets [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	48,657		30,455		20,499
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	0		0		0
Liabilities, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Commodity derivative instruments, liabilities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Liabilities, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Interest rate swap [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Liabilities, fair value disclosure	0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Money market funds [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Insurance investment contract [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	0	[1]	0	[2]	0	[3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Auction rate securities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure			0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Collateralized mortgage backed securities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | US Treasury securities [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	0		0		0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 [Member] | Commodity derivative instruments , assets [Member]
Fair value, assets and liabilities measured on recurring and nonrecurring basis [Line Items]
Assets, fair value disclosure	0		0		0

[1] The insurance investment contract invests approximately 28 percent in common stock of mid-cap companies, 28 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies and 15 percent in fixed-income and other investments.

[2] The insurance investment contract invests approximately 33 percent in common stock of mid-cap companies, 34 percent in common stock of small-cap companies, 32 percent in common stock of large-cap companies and 1 percent in cash and cash equivalents.

[3] The insurance investment contract invests approximately 34 percent in common stock of mid-cap companies, 33 percent in common stock of small-cap companies, 32 percent in common stock of large-cap companies and 1 percent in cash and cash equivalents.