Investments - Credit loss impairments on fixed income securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Investments, Debt and Equity Securities [Abstract]
Balance, beginning of period	$ 202,862	$ 299,720	$ 268,707	$ 333,379
Credit loss impairment recognized in the current period on securities not previously impaired	242	2,805	769	8,683
Credit loss impairments previously recognized on securities which matured, paid down, prepaid or were sold during the period	(17,341)	(33,114)	(78,960)	(87,092)
Credit loss impairments previously recognized on securities impaired to fair value during the period	0	0	0	(12,891)
Additional credit loss impairments recognized in the current period on securities previously impaired	2,942	4,834	7,242	41,716
Accretion of credit loss impairments previously recognized due to an increase in cash flows expected to be collected	(2,222)	(2,773)	(11,275)	(12,323)
Balance, end of period	$ 186,483	$ 271,472	$ 186,483	$ 271,472