Retirement Plan Benefits - Pension Plan Investments at Fair Value Represented 5% or More of Pension Plan Net Assets (Details) (Pension Plan Net Assets, Credit Concentration Risk [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]
Pension plan investments, at fair value	$ 375,679	$ 336,287
EB Daily Valued Small Cap Stock Index Fund
Defined Benefit Plan Disclosure [Line Items]
Pension plan investments, at fair value	25,842	23,038
EB Daily Valued Stock Index Fund
Defined Benefit Plan Disclosure [Line Items]
Pension plan investments, at fair value	85,834	82,060
EB Daily Valued International Stock Index Fund
Defined Benefit Plan Disclosure [Line Items]
Pension plan investments, at fair value	25,850	21,938
EB Long Term Government Bond Index
Defined Benefit Plan Disclosure [Line Items]
Pension plan investments, at fair value	45,732	44,058
EB Long Term Credit Bond Index
Defined Benefit Plan Disclosure [Line Items]
Pension plan investments, at fair value	67,902	60,111
Corporate Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Pension plan investments, at fair value	$ 124,519	$ 105,082