Investment Information - Narrative (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2014 lots		Dec. 31, 2013 lots		Mar. 31, 2013
Narrative items:
Investable assets	$ 15,344,386,000				$ 13,132,116,000
OTTI unrealized losses at current fair value	4,800,000		6,000,000
Continuous unrealized loss, qualitative disclosures:
Number of positions in an unrealized loss position (lots)	1,580		2,080
Total number of positions (lots)	4,310		4,400
Largest single loss	3,600,000		3,500,000
Securities lending, qualitative disclosures:
Fair value of fixed maturities and short-term investments pledged under securities lending agreements	100,600,000		105,100,000
Amortized cost of fixed maturities and short-term investments pledged under securities lending agreements	100,900,000		105,900,000
Fair value of securities backing securities lending program	96,264,000		100,584,000
Approximate fair value of sub-prime securities backing the securities lending program	6,300,000		6,300,000
OTTI Unrealized Losses	11,171,000	[1]	12,942,000	[1]
Underwriting segments
Narrative items:
Investable assets	14,261,106,000				13,132,116,000
Other
Narrative items:
Investable assets	$ 1,083,280,000				$ 0

[1] Represents the total other-than-temporary impairments (“OTTI”) recognized in accumulated other comprehensive income (“AOCI”). It does not include the change in fair value subsequent to the impairment measurement date. At March 31, 2014, the net unrealized gain related to securities for which a non-credit OTTI was recognized in AOCI was $4.8 million, compared to a net unrealized gain of $6.0 million at December 31, 2013.