Cash, Cash Equivalents and Investments - Summary of Available-For-Sale Investments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 355,029
Gross Unrealized Gains	1,295
Gross Unrealized Losses	(106)
Fair value	356,218
Corporate Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	136,864
Corporate Bonds [Member] | Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	44,935
Gross Unrealized Gains	94
Gross Unrealized Losses	(4)
Fair value	45,025
Corporate Bonds [Member] | Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	91,131
Gross Unrealized Gains	723
Gross Unrealized Losses	(15)
Fair value	91,839
Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	83,843
Municipal Bonds [Member] | Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	25,241
Gross Unrealized Gains	17
Gross Unrealized Losses	(1)
Fair value	25,257
Municipal Bonds [Member] | Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	58,246
Gross Unrealized Gains	341
Gross Unrealized Losses	(1)
Fair value	58,586
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	47,202
Asset-Backed Securities [Member] | Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,192
Gross Unrealized Gains	5
Gross Unrealized Losses	(13)
Fair value	12,184
Asset-Backed Securities [Member] | Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	34,998
Gross Unrealized Gains	48
Gross Unrealized Losses	(28)
Fair value	35,018
Treasury Bills [Member] | Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,044
Gross Unrealized Losses	(1)
Fair value	10,043
Treasury Bills [Member] | Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	35,993
Gross Unrealized Gains	2
Gross Unrealized Losses	(6)
Fair value	35,989
Covered Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	7,421
Covered Bonds [Member] | Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,679
Gross Unrealized Losses	(15)
Fair value	4,664
Covered Bonds [Member] | Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,753
Gross Unrealized Gains	4
Fair value	2,757
Agency Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	27,023
Agency Bonds [Member] | Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,999
Gross Unrealized Losses	(1)
Fair value	2,998
Agency Bonds [Member] | Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,001
Gross Unrealized Gains	43
Gross Unrealized Losses	(19)
Fair value	24,025
Sovereign Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value	7,833
Sovereign Bonds [Member] | Short-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,699
Fair value	1,699
Sovereign Bonds [Member] | Long-term investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	6,118
Gross Unrealized Gains	18
Gross Unrealized Losses	(2)
Fair value	$ 6,134