Derivative Instruments (Composition of Credit Derivatives Written Credit Protection) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Credit Derivatives [Line Items]
Notional	$ 315,000	$ 369,000
Fair value	411	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	29	101
Transportation [Member]
Credit Derivatives [Line Items]
Notional	25,000	25,000
Fair value	30	106
Basic Industry [Member]
Credit Derivatives [Line Items]
Notional	40,000	40,000
Fair value	68	248
Capital Goods [Member]
Credit Derivatives [Line Items]
Notional	90,000	90,000
Fair value	109	315
Consumer Cyclical [Member]
Credit Derivatives [Line Items]
Notional	20,000	20,000
Fair value	29	120
Consumer Non-cyclical [Member]
Credit Derivatives [Line Items]
Notional	120,000	120,000
Fair value	$ 146	$ 655