Fair Value Measurements and Derivative Instruments Fair Value Measurements and Derivative Instruments Available For Sale Securities (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Document Period End Date	Jun. 30, 2013
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis	$ 54,032
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	54,085
Available-for-sale Securities, Amortized Cost Basis	361,995	375,693
Available-for-sale Securities, Gross Unrealized Gains	5,713	10,216
Available-for-sale Securities, Gross Unrealized Losses	(6,310)	(3,221)
Available-for-sale Securities	361,398	382,688
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis	38,948
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Fair Value	40,028
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Amortized Cost Basis	83,026
Available-for-sale Securities, Debt Maturities, Year Six Through Ten, Fair Value	81,492
Available-for-sale Securities, Debt Maturities, after Ten Years, Amortized Cost Basis	185,989
Available-for-sale Securities, Debt Maturities, after Ten Years, Fair Value	185,793
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	361,995
Available-for-sale Securities, Debt Securities	361,398
Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	26,616	44,611
Available-for-sale Securities, Gross Unrealized Gains	0	53
Available-for-sale Securities, Gross Unrealized Losses	(6)	0
Available-for-sale Securities	26,610	44,664
US Treasury and Government [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	116,570	127,807
Available-for-sale Securities, Gross Unrealized Gains	2,045	3,972
Available-for-sale Securities, Gross Unrealized Losses	(3,773)	(56)
Available-for-sale Securities	114,842	131,723
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	39,793	41,095
Available-for-sale Securities, Gross Unrealized Gains	1,505	2,851
Available-for-sale Securities, Gross Unrealized Losses	(449)	(20)
Available-for-sale Securities	40,849	43,926
Collateralized Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	179,016	162,180
Available-for-sale Securities, Gross Unrealized Gains	2,163	3,340
Available-for-sale Securities, Gross Unrealized Losses	(2,082)	(3,145)
Available-for-sale Securities	$ 179,097	$ 162,375