Investments Amortized Cost and Estimated Fair Value of Investments in Available-for-sale Securities, by Contractual Maturity (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013
Cost
Mature in one year or less	$ 40,580
Mature after one year through three years	21,097
Mature in more than three years	21,833
Total debt securities	83,510
Securities with no contractual maturities	146,438
Total	229,948
Estimated Fair Value
Mature in one year or less	40,617
Mature after one year through three years	21,251
Mature in more than three years	22,274
Total debt securities	84,142
Securities with no contractual maturities	156,252
Total	$ 240,394