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Available-for-sale securities (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Schedule of available-for-sale securities [Line Items]
Investments used to satisfy obligations under nonqualified benefit plans	$ 48,400,000	$ 33,600,000	$ 48,400,000	$ 33,600,000	$ 38,400,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	2,400,000	(6,700,000)	4,700,000	(5,900,000)
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	47,399,000		47,399,000		53,109,000
Available-for-sale securities, gross unrealized gains	11,356,000		11,356,000		6,600,000
Available-for-sale securities, gross unrealized losses	(2,000)		(2,000)		(5,000)
Available-for-sale securities	58,753,000		58,753,000		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	11,134,000		11,134,000		6,468,000
Available-for-sale securities, gross unrealized losses	0		0		0
Available-for-sale securities	48,384,000		48,384,000		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	8,391,000		8,391,000		8,206,000
Available-for-sale securities, gross unrealized gains	175,000		175,000		95,000
Available-for-sale securities, gross unrealized losses	(2,000)		(2,000)		(5,000)
Available-for-sale securities	8,564,000		8,564,000		8,296,000
US Treasury securities [Member]
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	1,758,000		1,758,000		1,619,000
Available-for-sale securities, gross unrealized gains	47,000		47,000		37,000
Available-for-sale securities, gross unrealized losses	0		0		0
Available-for-sale securities	$ 1,805,000		$ 1,805,000		$ 1,656,000

X

- Definition

Investments used to satisfy obligations under unfunded, nonqualified benefit plans for executive officers and certain key management employees.

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Net unrealized gain (loss) of investments used to satify obligations under unfunded, nonqualified benefit plans for executive officers and certain key management employees.

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- Definition

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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