Comprehensive Income (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Comprehensive Income
Net income	$ 55,686	$ 52,900	$ 107,614	$ 100,718
Change in net unrealized gain/loss during the period	82,674	56,982	90,964	67,865
Reclassification adjustment for gains in net income		(1)	(5)	(6)
Change in the net actuarial gain/loss on defined benefit post-retirement benefit plans	782	729	1,565	1,457
Change in the accumulated gain/loss on effective cash flow hedging derivatives	(9,961)	21,921	(18,155)	25,390
Other comprehensive income (loss), before tax, total	73,495	79,631	74,369	94,706
Change in net unrealized gain/loss during the period, tax	28,936	19,944	31,838	23,753
Reclassification adjustment for gains in net income, tax			(2)	(2)
Change in the net actuarial gain/loss on defined benefit post- retirement benefit plans, tax	274	255	548	510
Change in the accumulated gain/loss on effective cash flow hedging derivatives, tax	(3,486)	7,672	(6,354)	8,886
Deferred tax expense	25,724	27,871	26,030	33,147
Other comprehensive income, net of tax	47,771	51,760	48,339	61,559
Total comprehensive income	$ 103,457	$ 104,660	$ 155,953	$ 162,277