Composition of Investments (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
US Government Agencies Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	$ 2,077,989	$ 2,190,957
Available For Sale Securities Unrealized Gains	8,548	10,225
Available For Sale Securities Unrealized Losses	(1,451)	(967)
Available For Sale Securities Fair Value	2,085,086	2,200,215
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	6,536,966	6,534,797
Available For Sale Securities Unrealized Gains	63,969	66,729
Available For Sale Securities Unrealized Losses	(7,251)	(6,436)
Available For Sale Securities Fair Value	6,593,684	6,595,090
Publicly Traded Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	70,374	46,665
Available For Sale Securities Unrealized Gains	40,313	40,548
Available For Sale Securities Unrealized Losses	(616)	(731)
Available For Sale Securities Fair Value	110,071	86,482
Debt and Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	6,607,340	6,581,462
Available For Sale Securities Unrealized Gains	104,282	107,277
Available For Sale Securities Unrealized Losses	(7,867)	(7,167)
Available For Sale Securities Fair Value	6,703,755	6,681,572
US Corporate Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	1,512,318	1,480,026
Available For Sale Securities Unrealized Gains	8,924	10,189
Available For Sale Securities Unrealized Losses	(436)	(509)
Available For Sale Securities Fair Value	1,520,806	1,489,706
High Yield Corporate Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	496,222	485,799
Available For Sale Securities Unrealized Gains	33,603	34,223
Available For Sale Securities Unrealized Losses	(766)	(800)
Available For Sale Securities Fair Value	529,059	519,222
Asset-backed Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	3,731	2,115
Available For Sale Securities Unrealized Gains	3	3
Available For Sale Securities Unrealized Losses	(4)	0
Available For Sale Securities Fair Value	3,730	2,118
Municipal Obligations
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	966,539	1,032,320
Available For Sale Securities Unrealized Gains	4,799	3,007
Available For Sale Securities Unrealized Losses	(45)	(540)
Available For Sale Securities Fair Value	971,293	1,034,787
Auction Rate Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	72,076	73,545
Available For Sale Securities Unrealized Gains	0	0
Available For Sale Securities Unrealized Losses	(3,829)	(3,365)
Available For Sale Securities Fair Value	68,247	70,180
Foreign Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Amortized Cost	1,408,091	1,270,035
Available For Sale Securities Unrealized Gains	8,092	9,082
Available For Sale Securities Unrealized Losses	(720)	(255)
Available For Sale Securities Fair Value	$ 1,415,463	$ 1,278,862

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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+ Details

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References+ Details

- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References+ Details