Investments - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012 Security	Sep. 30, 2011	Sep. 30, 2012 Security	Sep. 30, 2011	Dec. 31, 2011 Security
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential and commercial mortgage-backed securities	$ 3,544,088,000		$ 3,544,088,000		$ 3,335,199,000
Number of securities classified as available-for-sale in an unrealized loss position	37		37		107
Fair value of securities classified as available-for-sale in an unrealized loss position	41,454,000		41,454,000		156,526,000
Number of securities classified as available-for-sale in an unrealized loss position for twelve months or longer	25		25		59
Other investments recorded in private equities	389,728,000		389,728,000		192,264,000
Fair value of changes in assets included in earnings	300,000	(300,000)	8,100,000	6,100,000
Fair value of changes in assets included in net realized and unrealized gains	300,000	(300,000)	8,100,000	6,100,000
Residential mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential and commercial mortgage-backed securities	124,353,000		124,353,000		110,785,000
Commercial mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential and commercial mortgage-backed securities	137,161,000		137,161,000		86,694,000
Mortgage-backed securities, Issued by US government sponsored enterprises [Member] | Residential mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential and commercial mortgage-backed securities	3,889,000	4,624,000	3,889,000	4,624,000
Mortgage-backed securities, Issued by US government sponsored enterprises [Member] | Commercial mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential and commercial mortgage-backed securities
Real estate debt fund [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments recorded in private equities	15,861,000		15,861,000
Redemption request			10 days
Fixed income hedge funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Imposed lock-up period	3 years
Redemption request			90 days
Private equity funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other investments recorded in private equities	$ 115,199,000		$ 115,199,000		$ 107,388,000
Percent of other investments recorded in private equities	2.60%		2.60%		2.40%