Schedule I Disclosures for Parent Company Only Schedule I, Condensed Statement of OCI (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Net income (loss) available for common stock	$ 114,962	$ 81,528	$ 49,730
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Benefit plan liability adjustments - net gain (loss) (net of tax of $(3,813), $296 and $4,135, respectively)	8,237	(542)	(7,609)
Benefit plan liability adjustments - prior service (costs) (net of tax of $185, $86 and $176, respectively)	(406)	(157)	(325)
Reclassification adjustment of benefit plan liability - net gain (loss) (net of tax of $(971))	1,820	0	0
Reclassification adjustment of benefit plan liability - prior service cost (net of tax of $88)	(165)	0	0
Fair value adjustment on derivatives designated as cash flow hedges (net of tax of $(2,445), $887 and $1,708, respectively)	4,534	(1,268)	(2,831)
Reclassification adjustment of cash flow hedges settled and included in net income (loss) (net of tax of $(2,016), $534 and $(709), respectively)	4,046	(643)	1,468
Other comprehensive income (loss), net of tax	18,066	(2,610)	(9,297)
Comprehensive income (loss)	133,028	78,918	40,433
Other Comprehensive Income (Loss), Tax, Parenthetical Disclosures [Abstract]
Benefit plan liability adjustments - net gain (loss) (net of tax of	(3,813)	296	4,135
Benefit plan liability adjustments - prior service (costs) (net of tax of	185	86	176
Reclassification adjustment of benefit plan liability - net gain (loss) tax	(971)	0	0
Reclassification adjustment of benefit plan liability - prior service cost, tax	88	0	0
Fair value adjustment on derivatives designated as cash flow hedges (net of tax of	(2,445)	887	1,708
Reclassification adjustment of cash flow hedges settled and included in net income (loss) (net of tax of	$ (2,016)	$ 534	$ (709)