Retirement Plan Benefits - Components of Change in Benefit Obligation of Pension Plans (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, at beginning of year	$ 482,609	$ 427,213
Interest cost	21,440	22,663	22,685
Benefits paid	(16,598)	(15,493)
Actuarial loss	51,815	48,226
Benefit obligation, at end of year	$ 539,266	$ 482,609	$ 427,213