Comprehensive income (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Comprehensive income [Abstract]
Net (loss) income	$ (165,205,000)	$ (51,528,000)	$ (350,598,000)	$ (177,068,000)
Other comprehensive income	38,416,000	58,899,000	67,257,000	79,073,000
Total comprehensive (loss) income	(126,789,000)	7,371,000	(283,341,000)	(97,995,000)
Other comprehensive income (net of tax) [Abstract]
Change in unrealized gains and losses on investments	48,437,000	47,607,000	72,754,000	74,931,000
Unrealized foreign currency translation adjustment	(10,021,000)	11,292,000	(5,497,000)	4,142,000
Other comprehensive income	38,416,000	58,899,000	67,257,000	79,073,000
Tax expense on other comprehensive income	20,300,000	31,700,000	35,600,000	42,100,000
Accumulated other comprehensive income (loss) [Abstract]
Accumulated other comprehensive income	89,393,000	22,100,000	89,393,000	22,100,000	22,136,000
Net unrealized gains on investments	105,300,000	32,500,000	105,300,000	32,500,000
Foreign currency translation adjustment	14,900,000	20,400,000	14,900,000	20,400,000
Defined benefit plans	$ 30,800,000	$ 30,800,000	$ 30,800,000	$ 30,800,000