Investment Securities (Details 3) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total	$ 347,567	$ 304,045
Level 1 [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total	70,947	43,177
Level 2 [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total	276,620	260,868
Level 3 [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total
U.S. Treasury securities [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total	250	250
U.S. Treasury securities [Member] | Level 1 [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total	250	250
U.S. Treasury securities [Member] | Level 3 [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total
Obligations of U.S. government sponsored enterprises [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total	130,674	124,636
Obligations of U.S. government sponsored enterprises [Member] | Level 1 [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total	40,727	42,927
Obligations of U.S. government sponsored enterprises [Member] | Level 2 [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total	89,947	81,709
Obligations of U.S. government sponsored enterprises [Member] | Level 3 [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total
Corporate debt securities [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total	133,536	132,862
Corporate debt securities [Member] | Level 2 [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total	133,536	132,862
Corporate debt securities [Member] | Level 3 [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total
Commercial Paper [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total	29,970
Commercial Paper [Member] | Level 1 [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total	29,970
Commercial Paper [Member] | Level 3 [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total
Foreign corporate debt securities [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total	33,580	27,229
Foreign corporate debt securities [Member] | Level 2 [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total	33,580	27,229
Foreign corporate debt securities [Member] | Level 3 [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total
Commercial mortgage-backed securities [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total	19,557	19,068
Commercial mortgage-backed securities [Member] | Level 2 [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total	19,557	19,068
Commercial mortgage-backed securities [Member] | Level 3 [Member]
Schedule of investments in accordance with fair value measurement
Fair value measurement of class of securities Total