Available-for-sale securities (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Schedule of available-for-sale securities [Line Items]
Investments used to satisfy obligations under nonqualified benefit plans	$ 46,000,000	$ 40,300,000	$ 46,000,000	$ 40,300,000	$ 38,400,000
Increase (decrease) in fair value of investments used to satisfy obligations under nonqualified benefit plans	(2,700,000)	(1,300,000)	2,200,000	790,000
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	47,338,000		47,338,000		53,109,000
Available-for-sale securities, gross unrealized gains	8,874,000		8,874,000		6,600,000
Available-for-sale securities, gross unrealized losses	(6,000)		(6,000)		(5,000)
Available-for-sale securities	56,206,000		56,206,000		59,704,000
Insurance investment contract [Member]
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	37,250,000		37,250,000		31,884,000
Available-for-sale securities, gross unrealized gains	8,709,000		8,709,000		6,468,000
Available-for-sale securities, gross unrealized losses	0		0		0
Available-for-sale securities	45,959,000		45,959,000		38,352,000
Auction rate securities [Member]
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis					11,400,000
Available-for-sale securities, gross unrealized gains					0
Available-for-sale securities, gross unrealized losses					0
Available-for-sale securities					11,400,000
Collateralized mortgage backed securities [Member]
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	8,130,000		8,130,000		8,206,000
Available-for-sale securities, gross unrealized gains	128,000		128,000		95,000
Available-for-sale securities, gross unrealized losses	(5,000)		(5,000)		(5,000)
Available-for-sale securities	8,253,000		8,253,000		8,296,000
US Treasury securities [Member]
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	1,958,000		1,958,000		1,619,000
Available-for-sale securities, gross unrealized gains	37,000		37,000		37,000
Available-for-sale securities, gross unrealized losses	(1,000)		(1,000)		0
Available-for-sale securities	$ 1,994,000		$ 1,994,000		$ 1,656,000