Defined Benfit Plan (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012	Dec. 31, 2012
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 1,239		$ 1,157
Interest cost	1,475		1,450
Other cost	1,208		1,166
Expected return on assets	(1,380)		(1,148)
Net periodic benefit cost	2,542		2,625
Plan Assets [Abstract]
Fair value of plan assets	81,825			79,020
Employer Contributions [Abstract]
Estimated employer contributions for the current fiscal year	10,300
Quoted Prices in Active Markets for Identical Assets, Level 1 [Member]
Plan Assets [Abstract]
Fair value of plan assets	40,505			40,137
Significant Observable Inputs, Level 2 [Member]
Plan Assets [Abstract]
Fair value of plan assets	41,320			38,883
Significant Unobservable Inputs, Level 3 [Member]
Plan Assets [Abstract]
Fair value of plan assets	0			0
Equity Securities [Member]
Plan Assets [Abstract]
Target plan asset allocations	55.00%		55.00%
Fixed Income Securities [Member]
Plan Assets [Abstract]
Target plan asset allocations	45.00%		45.00%
Cash and cash equivalents [Member]
Plan Assets [Abstract]
Fair value of plan assets	3,146			7,056
Cash and cash equivalents [Member] | Quoted Prices in Active Markets for Identical Assets, Level 1 [Member]
Plan Assets [Abstract]
Fair value of plan assets	3,146			7,056
Cash and cash equivalents [Member] | Significant Observable Inputs, Level 2 [Member]
Plan Assets [Abstract]
Fair value of plan assets	0			0
Cash and cash equivalents [Member] | Significant Unobservable Inputs, Level 3 [Member]
Plan Assets [Abstract]
Fair value of plan assets	0			0
Actively Managed [Member] | U.S. Large Cap Equity [Member]
Plan Assets [Abstract]
Fair value of plan assets	26,062	[1]		22,749	[1]
Actively Managed [Member] | U.S. Large Cap Equity [Member] | Quoted Prices in Active Markets for Identical Assets, Level 1 [Member]
Plan Assets [Abstract]
Fair value of plan assets	26,062	[1]		22,749	[1]
Actively Managed [Member] | U.S. Large Cap Equity [Member] | Significant Observable Inputs, Level 2 [Member]
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]
Actively Managed [Member] | U.S. Large Cap Equity [Member] | Significant Unobservable Inputs, Level 3 [Member]
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]
Actively Managed [Member] | U.S. Mid Cap Equity [Member]
Plan Assets [Abstract]
Fair value of plan assets	4,479	[1]		3,989	[1]
Actively Managed [Member] | U.S. Mid Cap Equity [Member] | Quoted Prices in Active Markets for Identical Assets, Level 1 [Member]
Plan Assets [Abstract]
Fair value of plan assets	4,479	[1]		3,989	[1]
Actively Managed [Member] | U.S. Mid Cap Equity [Member] | Significant Observable Inputs, Level 2 [Member]
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]
Actively Managed [Member] | U.S. Mid Cap Equity [Member] | Significant Unobservable Inputs, Level 3 [Member]
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]
Actively Managed [Member] | U.S. Small Cap Equity [Member]
Plan Assets [Abstract]
Fair value of plan assets	2,588	[1]		2,174	[1]
Actively Managed [Member] | U.S. Small Cap Equity [Member] | Quoted Prices in Active Markets for Identical Assets, Level 1 [Member]
Plan Assets [Abstract]
Fair value of plan assets	2,588	[1]		2,174	[1]
Actively Managed [Member] | U.S. Small Cap Equity [Member] | Significant Observable Inputs, Level 2 [Member]
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]
Actively Managed [Member] | U.S. Small Cap Equity [Member] | Significant Unobservable Inputs, Level 3 [Member]
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]
Actively Managed [Member] | Non-U.S. Large Cap Equity [Member]
Plan Assets [Abstract]
Fair value of plan assets	4,230	[1]		4,169	[1]
Actively Managed [Member] | Non-U.S. Large Cap Equity [Member] | Quoted Prices in Active Markets for Identical Assets, Level 1 [Member]
Plan Assets [Abstract]
Fair value of plan assets	4,230	[1]		4,169	[1]
Actively Managed [Member] | Non-U.S. Large Cap Equity [Member] | Significant Observable Inputs, Level 2 [Member]
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]
Actively Managed [Member] | Non-U.S. Large Cap Equity [Member] | Significant Unobservable Inputs, Level 3 [Member]
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]
Passive Index Fund ETFs [Member] | REIT [Member]
Plan Assets [Abstract]
Fair value of plan assets	3,981	[1]		3,792	[1]
Passive Index Fund ETFs [Member] | REIT [Member] | Quoted Prices in Active Markets for Identical Assets, Level 1 [Member]
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]
Passive Index Fund ETFs [Member] | REIT [Member] | Significant Observable Inputs, Level 2 [Member]
Plan Assets [Abstract]
Fair value of plan assets	3,981	[1]		3,792	[1]
Passive Index Fund ETFs [Member] | REIT [Member] | Significant Unobservable Inputs, Level 3 [Member]
Plan Assets [Abstract]
Fair value of plan assets	0	[1]		0	[1]
Fixed Income [Member]
Plan Assets [Abstract]
Fair value of plan assets	37,339	[2]		35,091	[2]
Fixed Income [Member] | Quoted Prices in Active Markets for Identical Assets, Level 1 [Member]
Plan Assets [Abstract]
Fair value of plan assets	0	[2]		0	[2]
Fixed Income [Member] | Significant Observable Inputs, Level 2 [Member]
Plan Assets [Abstract]
Fair value of plan assets	37,339	[2]		35,091	[2]
Fixed Income [Member] | Significant Unobservable Inputs, Level 3 [Member]
Plan Assets [Abstract]
Fair value of plan assets	$ 0	[2]		$ 0	[2]

[1] Actively managed portfolio of securities with the goal to exceed the stated benchmark performance.

[2] Actively managed portfolio of fixed income securities with the goal to exceed the Barclays 1-5 Year Government/Credit, Barclays Intermediate Government/Credit, and Merrill Lynch Preferred Stock Fixed Rate.