Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Balance at beginning of period			$ 367,499
Other comprehensive income (loss)	30,955	119,278	61,843	93,842
Balance at end of period	429,342		429,342
Net Unrealized Gains (Losses) on Investments
Balance at beginning of period	395,900	83,200	366,700	109,700
Other comprehensive income (loss)	32,300	119,000	61,500	92,500
Balance at end of period	428,200	202,200	428,200	202,200
Cumulative Translation Adjustment
Balance at beginning of period	(6,800)	(6,500)	(8,600)	(7,700)
Other comprehensive income (loss)	(1,200)	300	600	1,500
Balance at end of period	(8,000)	(6,200)	(8,000)	(6,200)
Cash Flow Hedge
Balance at beginning of period	9,300	9,700	9,400	9,800
Other comprehensive income (loss)	(100)	(100)	(200)	(200)
Balance at end of period	9,200	9,600	9,200	9,600
Total Accumulated Other Comprehensive Income
Balance at beginning of period	398,400	86,400	367,500	111,800
Other comprehensive income (loss)	31,000	119,200	61,843	93,800
Balance at end of period	$ 429,400	$ 205,600	$ 429,400	$ 205,600