Employee Benefit Plans (Details 3) (Pension Benefits, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Strategic allocation
Strategic allocation (as a percent)	100.00%
Actual allocation
Actual allocation (as a percent)	100.00%	100.00%
Fair value of the plan assets
Fair value of the pension trust's assets	$ 232,960	$ 202,537	$ 191,736
Cash and cash equivalents
Fair value of the plan assets
Fair value of the pension trust's assets	1,397	3,568
Equity investments
Strategic allocation
Equity investments (as a percent)	60.00%
Tactical Range
Equity investments, maximum (as a percent)	70.00%
Equity investments, minimum (as a percent)	50.00%
Actual allocation
Equity investments (as a percent)	60.20%	59.20%
U.S. large-cap funds
Strategic allocation
Equity investments (as a percent)	38.50%
Tactical Range
Equity investments, maximum (as a percent)	47.00%
Equity investments, minimum (as a percent)	30.00%
Actual allocation
Equity investments (as a percent)	37.30%	37.20%
Fair value of the plan assets
Fair value of the pension trust's assets	86,892	75,333
International fund
Strategic allocation
Equity investments (as a percent)	13.00%
Tactical Range
Equity investments, maximum (as a percent)	16.00%
Equity investments, minimum (as a percent)	10.00%
Actual allocation
Equity investments (as a percent)	13.30%	12.50%
Fair value of the plan assets
Fair value of the pension trust's assets	31,038	25,332
U.S. mid-cap fund
Strategic allocation
Equity investments (as a percent)	8.50%
Tactical Range
Equity investments, maximum (as a percent)	11.00%
Equity investments, minimum (as a percent)	6.00%
Actual allocation
Equity investments (as a percent)	9.60%	9.50%
Fair value of the plan assets
Fair value of the pension trust's assets	22,368	19,350
Fixed income investments
Strategic allocation
Fixed income investments (as a percent)	40.00%
Tactical Range
Fixed income investments, minimum (as a percent)	30.00%
Fixed income investments, maximum (as a percent)	50.00%
Actual allocation
Fixed income investments (as a percent)	39.20%	39.00%
Intermediate-term bond
Fair value of the plan assets
Fair value of the pension trust's assets	85,814	73,928
Long-term bond
Fair value of the plan assets
Fair value of the pension trust's assets	5,451	5,026
Cash
Strategic allocation
Cash (as a percent)	0.00%
Tactical Range
Cash, minimum (as a percent)	0.00%
Cash, maximum (as a percent)	5.00%
Actual allocation
Cash (as a percent)	0.60%	1.80%
Level 1(L1)
Fair value of the plan assets
Fair value of the pension trust's assets	232,960	202,537
Level 1(L1) | Cash and cash equivalents
Fair value of the plan assets
Fair value of the pension trust's assets	1,397	3,568
Level 1(L1) | U.S. large-cap funds
Fair value of the plan assets
Fair value of the pension trust's assets	86,892	75,333
Level 1(L1) | International fund
Fair value of the plan assets
Fair value of the pension trust's assets	31,038	25,332
Level 1(L1) | U.S. mid-cap fund
Fair value of the plan assets
Fair value of the pension trust's assets	22,368	19,350
Level 1(L1) | Intermediate-term bond
Fair value of the plan assets
Fair value of the pension trust's assets	85,814	73,928
Level 1(L1) | Long-term bond
Fair value of the plan assets
Fair value of the pension trust's assets	$ 5,451	$ 5,026