Derivative Instruments (Composition of Credit Derivatives Written Credit Protection) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Credit Derivatives [Line Items]
Notional	$ 315,000	$ 369,000
Fair value	805	1,640
Finance [Member]
Credit Derivatives [Line Items]
Notional	0	54,000
Fair value	0	95
Energy [Member]
Credit Derivatives [Line Items]
Notional	20,000	20,000
Fair value	58	101
Transportation [Member]
Credit Derivatives [Line Items]
Notional	25,000	25,000
Fair value	55	106
Basic Industry [Member]
Credit Derivatives [Line Items]
Notional	40,000	40,000
Fair value	132	248
Capital Goods [Member]
Credit Derivatives [Line Items]
Notional	90,000	90,000
Fair value	220	315
Consumer Cyclical [Member]
Credit Derivatives [Line Items]
Notional	20,000	20,000
Fair value	56	120
Consumer Non-cyclical [Member]
Credit Derivatives [Line Items]
Notional	120,000	120,000
Fair value	$ 284	$ 655