Fair Value of Financial Assets and Liabilities - Composition of Investments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Unrealized Gains		$ 0
U.S. government and agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,186,952	2,474,029
Unrealized Gains	10,224	12,420
Unrealized (Losses)	(967)	(1,488)
Aggregate Fair Value	2,196,209	2,484,961
U.S. corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,470,770	1,400,373
Unrealized Gains	10,189	9,953
Unrealized (Losses)	(509)	(2,573)
Aggregate Fair Value	1,480,450	1,407,753
High yield corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	476,989	442,723
Unrealized Gains	34,223	12,498
Unrealized (Losses)	(800)	(7,742)
Aggregate Fair Value	510,412	447,479
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,114	29,101
Unrealized Gains	3	72
Unrealized (Losses)	0	(25)
Aggregate Fair Value	2,117	29,148
Municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,020,422	814,657
Unrealized Gains	3,007	2,021
Unrealized (Losses)	(540)	(597)
Aggregate Fair Value	1,022,889	816,081
Auction rate securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	73,451	82,900
Unrealized Gains	0
Unrealized (Losses)	(3,365)	(8,304)
Aggregate Fair Value	70,086	74,596
Foreign debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,264,195	984,696
Unrealized Gains	9,083	5,185
Unrealized (Losses)	(255)	(2,807)
Aggregate Fair Value	1,273,023	987,074
Total fixed income securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,494,893	6,228,479
Unrealized Gains	66,729	42,149
Unrealized (Losses)	(6,436)	(23,536)
Aggregate Fair Value	6,555,186	6,247,092
Publicly traded equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48,526	58,199
Unrealized Gains	38,687	6,802
Unrealized (Losses)	(731)	0
Aggregate Fair Value	86,482	65,001
Total
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,543,419	6,286,678
Unrealized Gains	105,416	48,951
Unrealized (Losses)	(7,167)	(23,536)
Aggregate Fair Value	$ 6,641,668	$ 6,312,093

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