Investments - Credit loss impairments on fixed income securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Investments, Debt and Equity Securities [Abstract]
Balance, beginning of period	$ 268,708	$ 333,379
Credit loss impairment recognized in the current period on securities not previously impaired	500	1,835
Credit loss impairments previously recognized on securities which matured, paid down, prepaid or were sold during the period	(6,475)	(17,195)
Credit loss impairments previously recognized on securities impaired to fair value during the period	0	(12,891)
Additional credit loss impairments recognized in the current period on securities previously impaired	2,304	14,930
Accretion of credit loss impairments previously recognized due to an increase in cash flows expected to be collected	(4,971)	(5,253)
Balance, end of period	$ 260,066	$ 314,805

The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity).

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This item represents reductions in the amount of cumulative losses recognized in earnings for other than temporary impairments (OTTI) for which a portion of the impairment had been recognized in other comprehensive income (a component of shareholders' equity) due to a change in status of the impaired security because of a change in the entity's intent to either sell the impaired security or it has become more likely than not that the entity will be required to sell the security before recovery of its cost basis.

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