Fair Value Measurements and Derivative Instruments (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 316,748	$ 325,368
Gross unrealized gains	4,374	3,504
Gross unrealized losses	4,080	6,580
Estimated fair value	317,042	322,292
Amortized cost - Within 1 year	26,473
Estimated fair value - Within 1 year	26,539
Amortized cost - After 1 year through 5 years	58,545
Estimated fair value - After 1 year through 5 years	59,152
Amortized cost - After 5 years through 10 years	87,268
Estimated fair value - After 5 years through 10 years	87,343
Amortized cost - After 10 years	144,462
Estimated fair value - After 10 years	144,008
Amortized cost - Total	316,748
Estimated fair value - Total	317,042
Debt securities - U.S. and foreign governments, agencies and municipalities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	129,856	121,803
Gross unrealized gains	1,422	999
Gross unrealized losses	1,797	3,372
Estimated fair value	129,481	119,430
Debt securities - corporate
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	45,410	37,901
Gross unrealized gains	1,198	935
Gross unrealized losses	314	572
Estimated fair value	46,294	38,264
Mortgage-backed / asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	141,482	165,664
Gross unrealized gains	1,754	1,570
Gross unrealized losses	1,969	2,636
Estimated fair value	$ 141,267	$ 164,598