Derivative Instruments (Composition of Credit Derivatives Written Credit Protection) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Credit Derivatives [Line Items]
Notional	$ 0	$ 315,000
Fair value	0	411
Energy [Member]
Credit Derivatives [Line Items]
Notional	0	20,000
Fair value	0	29
Transportation [Member]
Credit Derivatives [Line Items]
Notional	0	25,000
Fair value	0	30
Basic Industry [Member]
Credit Derivatives [Line Items]
Notional	0	40,000
Fair value	0	68
Capital Goods [Member]
Credit Derivatives [Line Items]
Notional	0	90,000
Fair value	0	109
Consumer Cyclical [Member]
Credit Derivatives [Line Items]
Notional	0	20,000
Fair value	0	29
Consumer Non-cyclical [Member]
Credit Derivatives [Line Items]
Notional	0	120,000
Fair value	$ 0	$ 146