Investments - Classification (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Investment [Line Items]
Total investments, fair value	$ 156,077		$ 109,319
Total fixed maturity securities [Member]
Investment [Line Items]
Marketable securities	150,182		105,665
U.S. treasury notes [Member]
Investment [Line Items]
Marketable securities	10,128		60,786
Agency bonds [Member]
Investment [Line Items]
Marketable securities	37,961		31,157
Commercial paper [Member]
Investment [Line Items]
Marketable securities	39,987		5,000
Certificates of deposit [Member]
Investment [Line Items]
Marketable securities	7,786		5,494
Municipal bonds [Member]
Investment [Line Items]
Marketable securities	49,319		3,228
Corporate Debt Securities [Member]
Investment [Line Items]
Marketable securities	5,001		0
Equity securities - mutual funds [Member]
Investment [Line Items]
Marketable securities	5,287		3,046
Cost-method investments [Member]
Investment [Line Items]
Cost method investments	608	[1]	608	[1]
Short-term investments [Member]
Investment [Line Items]
Total investments, fair value	108,191		79,532
Short-term investments [Member] | Total fixed maturity securities [Member]
Investment [Line Items]
Marketable securities	108,191		79,532
Short-term investments [Member] | U.S. treasury notes [Member]
Investment [Line Items]
Marketable securities	10,128		60,786
Short-term investments [Member] | Agency bonds [Member]
Investment [Line Items]
Marketable securities	10,995		10,148
Short-term investments [Member] | Commercial paper [Member]
Investment [Line Items]
Marketable securities	39,987		5,000
Short-term investments [Member] | Certificates of deposit [Member]
Investment [Line Items]
Marketable securities	5,802		3,598
Short-term investments [Member] | Municipal bonds [Member]
Investment [Line Items]
Marketable securities	41,279		0
Short-term investments [Member] | Corporate Debt Securities [Member]
Investment [Line Items]
Marketable securities	0		0
Short-term investments [Member] | Equity securities - mutual funds [Member]
Investment [Line Items]
Marketable securities	0		0
Short-term investments [Member] | Cost-method investments [Member]
Investment [Line Items]
Cost method investments	0		0
Long-term investments [Member]
Investment [Line Items]
Total investments, fair value	47,278		29,179
Long-term investments [Member] | Total fixed maturity securities [Member]
Investment [Line Items]
Marketable securities	41,991		26,133
Long-term investments [Member] | U.S. treasury notes [Member]
Investment [Line Items]
Marketable securities	0		0
Long-term investments [Member] | Agency bonds [Member]
Investment [Line Items]
Marketable securities	26,966		21,009
Long-term investments [Member] | Commercial paper [Member]
Investment [Line Items]
Marketable securities	0		0
Long-term investments [Member] | Certificates of deposit [Member]
Investment [Line Items]
Marketable securities	1,984		1,896
Long-term investments [Member] | Municipal bonds [Member]
Investment [Line Items]
Marketable securities	8,040		3,228
Long-term investments [Member] | Corporate Debt Securities [Member]
Investment [Line Items]
Marketable securities	5,001		0
Long-term investments [Member] | Equity securities - mutual funds [Member]
Investment [Line Items]
Marketable securities	5,287		3,046
Long-term investments [Member] | Cost-method investments [Member]
Investment [Line Items]
Cost method investments	0		0
Other assets [Member]
Investment [Line Items]
Total investments, fair value	608		608
Other assets [Member] | Total fixed maturity securities [Member]
Investment [Line Items]
Marketable securities	0		0
Other assets [Member] | U.S. treasury notes [Member]
Investment [Line Items]
Marketable securities	0		0
Other assets [Member] | Agency bonds [Member]
Investment [Line Items]
Marketable securities	0		0
Other assets [Member] | Commercial paper [Member]
Investment [Line Items]
Marketable securities	0		0
Other assets [Member] | Certificates of deposit [Member]
Investment [Line Items]
Marketable securities	0		0
Other assets [Member] | Municipal bonds [Member]
Investment [Line Items]
Marketable securities	0		0
Other assets [Member] | Corporate Debt Securities [Member]
Investment [Line Items]
Marketable securities	0		0
Other assets [Member] | Equity securities - mutual funds [Member]
Investment [Line Items]
Marketable securities	0		0
Other assets [Member] | Cost-method investments [Member]
Investment [Line Items]
Cost method investments	$ 608		$ 608

[1] Investments for which it is not practical to estimate fair value are included at cost and primarily consist of investments in privately-held companies.