Pension Benefit and Other Postretirement Benefit Plans - Assets Carried at Fair Value by Level (Details) (Pension Benefits [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Apr. 30, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Pension assets	$ 157,379	$ 156,741	$ 143,540
Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		106,619	98,269
Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		33,530	29,869
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		16,592	15,402
Estimate of Fair Value, Fair Value Disclosure [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		156,741	143,540
Pooled separate accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		33,530	29,869
Pooled separate accounts [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		0	0
Pooled separate accounts [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		33,530	29,869
Pooled separate accounts [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		0	0
Pooled separate accounts [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		33,530	29,869
Mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		106,619	98,269
Mutual funds [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		106,619	98,269
Mutual funds [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		0	0
Mutual funds [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		0	0
Mutual funds [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		106,619	98,269
Guaranteed deposit account [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		16,592	15,402
Guaranteed deposit account [Member] | Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		0	0
Guaranteed deposit account [Member] | Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		0	0
Guaranteed deposit account [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		16,592	15,402
Guaranteed deposit account [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension assets		$ 16,592	$ 15,402