Comprehensive Income (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Comprehensive Income [Abstract]
Net income	$ 54,507	$ 54,995	$ 162,121	$ 155,713
Change in net unrealized gain/loss during the period	78,552	45,587	169,516	113,452
Reclassification adjustment for gains in net income	(6,409)		(6,414)	(6)
Change in the net actuarial gain/loss on defined benefit post-retirement benefit plans	783	729	2,348	2,186
Change in the accumulated gain/loss on effective cash flow hedging derivatives	(9,136)	8,263	(27,291)	33,653
Other comprehensive income (loss), before tax, total	63,790	54,579	138,159	149,285
Change in net unrealized gain/loss during the period, tax	27,493	15,955	59,331	39,708
Reclassification adjustment for gains in net income, tax	(2,243)		(2,245)	(2)
Change in the net actuarial gain/loss on defined benefit post- retirement benefit plans, tax	274	255	822	765
Change in the accumulated gain/loss on effective cash flow hedging derivatives, tax	(3,198)	2,893	(9,552)	11,779
Deferred tax expense	22,326	19,103	48,356	52,250
Other comprehensive income, net of tax	41,464	35,476	89,803	97,035
Total comprehensive income	$ 95,971	$ 90,471	$ 251,924	$ 252,748