Consolidated Statement of Comprehensive Income (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, Portion Attributable to Parent, Parenthetical Disclosures [Abstract]
Benefit plan liability adjustments - net gain (loss), Tax	(3,813)	296	4,135
Benefit plan liability adjustments - prior service (costs), Tax	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, Tax	(2,445)	887	1,708
Reclassification adjustment of cash flow hedges settled and included in net income (loss), Tax	$ (2,016)	$ 534	$ (709)