Other Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Balance at beginning of period	$ 367,499
Other comprehensive income (loss)	30,888	(25,436)
Balance at end of period	398,387
Net Unrealized Gains (Losses) on Investments
Balance at beginning of period	366,700	109,700
Other comprehensive income (loss)	29,243	(26,574)
Balance at end of period	395,900	83,200
Cumulative Translation Adjustment
Balance at beginning of period	(8,600)	(7,700)
Other comprehensive income (loss)	1,750	1,243
Balance at end of period	(6,800)	(6,500)
Cash Flow Hedge
Balance at beginning of period	9,400	9,800
Other comprehensive income (loss)	(105)	(105)
Balance at end of period	9,300	9,700
Total Accumulated Other Comprehensive Income
Balance at beginning of period	367,500	111,800
Other comprehensive income (loss)	30,888	(25,400)
Balance at end of period	$ 398,400	$ 86,400