Cash, Cash Equivalents and Investments (Details 1) (USD $) In Thousands, unless otherwise specified	Apr. 01, 2012	Dec. 31, 2011
Summary of available-for-sale investments
Amortized Cost	$ 335,599
Gross Unrealized Gains	1,846
Gross Unrealized Losses	(204)
Fair Value, Short-term investments	159,209	144,431
Fair Value, Total	337,241
Treasury bills [Member]
Summary of available-for-sale investments
Fair Value, Short-term investments		1,945
Fair Value, Long-term investments	999
Fair Value, Total	999
Treasury bills [Member] | Long-term investments [Member]
Summary of available-for-sale investments
Amortized Cost	998
Gross Unrealized Gains	1
Fair Value, Long-term investments	999
Municipal bonds [Member]
Summary of available-for-sale investments
Fair Value, Short-term investments	57,862	54,036
Fair Value, Long-term investments	67,532	69,680
Fair Value, Total	125,394
Municipal bonds [Member] | Short-term investments [Member]
Summary of available-for-sale investments
Amortized Cost	57,774
Gross Unrealized Gains	89
Gross Unrealized Losses	(1)
Fair Value, Short-term investments	57,862
Municipal bonds [Member] | Long-term investments [Member]
Summary of available-for-sale investments
Amortized Cost	67,265
Gross Unrealized Gains	295
Gross Unrealized Losses	(28)
Fair Value, Long-term investments	67,532
Corporate bonds [Member]
Summary of available-for-sale investments
Fair Value, Short-term investments	61,517	55,570
Fair Value, Long-term investments	71,969	59,902
Fair Value, Total	133,486
Corporate bonds [Member] | Short-term investments [Member]
Summary of available-for-sale investments
Amortized Cost	61,459
Gross Unrealized Gains	188
Gross Unrealized Losses	(130)
Fair Value, Short-term investments	61,517
Corporate bonds [Member] | Long-term investments [Member]
Summary of available-for-sale investments
Amortized Cost	71,175
Gross Unrealized Gains	795
Gross Unrealized Losses	(1)
Fair Value, Long-term investments	71,969
Agency bonds [Member]
Summary of available-for-sale investments
Fair Value, Short-term investments	27,298	27,545
Fair Value, Long-term investments	13,987	12,335
Fair Value, Total	41,285
Agency bonds [Member] | Short-term investments [Member]
Summary of available-for-sale investments
Amortized Cost	27,220
Gross Unrealized Gains	87
Gross Unrealized Losses	(9)
Fair Value, Short-term investments	27,298
Agency bonds [Member] | Long-term investments [Member]
Summary of available-for-sale investments
Amortized Cost	13,988
Gross Unrealized Gains	34
Gross Unrealized Losses	(35)
Fair Value, Long-term investments	13,987
Sovereign Bonds [Member]
Summary of available-for-sale investments
Fair Value, Short-term investments	6,222	4,016
Fair Value, Long-term investments	18,279	22,355
Fair Value, Total	24,501
Sovereign Bonds [Member] | Short-term investments [Member]
Summary of available-for-sale investments
Amortized Cost	6,192
Gross Unrealized Gains	30
Fair Value, Short-term investments	6,222
Sovereign Bonds [Member] | Long-term investments [Member]
Summary of available-for-sale investments
Amortized Cost	18,019
Gross Unrealized Gains	260
Fair Value, Long-term investments	18,279
Covered bonds [Member]
Summary of available-for-sale investments
Fair Value, Short-term investments	4,443	1,319
Fair Value, Long-term investments	1,740	4,701
Fair Value, Total	6,183
Covered bonds [Member] | Short-term investments [Member]
Summary of available-for-sale investments
Amortized Cost	4,402
Gross Unrealized Gains	41
Fair Value, Short-term investments	4,443
Covered bonds [Member] | Long-term investments [Member]
Summary of available-for-sale investments
Amortized Cost	1,722
Gross Unrealized Gains	18
Fair Value, Long-term investments	1,740
Asset-backed securities [Member]
Summary of available-for-sale investments
Fair Value, Short-term investments	1,867
Fair Value, Long-term investments	3,526
Fair Value, Total	5,393
Asset-backed securities [Member] | Short-term investments [Member]
Summary of available-for-sale investments
Amortized Cost	1,865
Gross Unrealized Gains	2
Fair Value, Short-term investments	1,867
Asset-backed securities [Member] | Long-term investments [Member]
Summary of available-for-sale investments
Amortized Cost	3,520
Gross Unrealized Gains	6
Fair Value, Long-term investments	$ 3,526