Equity Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Prior service cost	$ (556)		$ (556)		$ (616)
Transition asset	162		162		195
Unrecognized net actuarial loss	(7,816)		(7,816)		(7,992)
Net income	59,942	52,951	116,674	106,881
Amortization of deferred pension costs, net of tax			203
Total accumulated other comprehensive income (loss)	$ (8,210)		$ (8,210)		$ (8,413)