Investments (Details 1) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Contractual maturities of corporate and municipal fixed-rate bonds
Total	$ 467,561	$ 440,982
Corporate bonds [Member]
Contractual maturities of corporate and municipal fixed-rate bonds
Less than one year	39,614
One to two years	114,900
Two to three years	29,221
Total	183,735	156,753
Municipal fixed-rate bonds [Member]
Contractual maturities of corporate and municipal fixed-rate bonds
Less than one year	83,045
One to two years	56,268
Two to three years	7,210
Three to five years	42,995
Total	$ 189,518	$ 174,826