Investments - Amortized cost and estimated fair value of investments in available-for-sale securities, by contractual maturity (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014
Cost
Mature in one year or less	$ 9,029
Mature after one year through three years	21,681
Mature in more than three years	33,627
Total debt securities	64,337
Securities with no contractual maturities	162,029
Total	226,366
Estimated Fair Value
Mature in one year or less	9,030
Mature after one year through three years	21,812
Mature in more than three years	35,160
Total debt securities	66,002
Securities with no contractual maturities	176,898
Total	$ 242,900