Fair value measurements (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011		Mar. 31, 2011
Concentration risks, percentage [Abstract]
Percentage investment in common stock of mid-cap companies	29.00%		33.00%		34.00%
Percentage investment in common stock of small-cap companies	29.00%		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	13.00%
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	107,412		187,659		154,834
Liabilities, fair value disclosure	29,651		18,863		58,581
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	26,330		14,101		58,581
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	3,321		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	9,942		97,500		75,658
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	48,704	[1]	38,352	[2]	41,594	[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		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	8,070		8,296		8,064
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	2,016		1,656		1,720
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	27,280		30,455		16,398
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		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	107,412		187,659		154,834
Liabilities, fair value disclosure	29,651		18,863		58,581
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	26,330		14,101		58,581
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	3,321		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	9,942		97,500		75,658
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	48,704	[1]	38,352	[2]	41,594	[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		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	8,070		8,296		8,064
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	2,016		1,656		1,720
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	27,280		30,455		16,398
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		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 29 percent in common stock of mid-cap companies, 29 percent in common stock of small-cap companies, 29 percent in common stock of large-cap companies and 13 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.