Debt Term Credit Agreement Narrative (Details) (Term Credit Agreement borrowings due 2019 at a variable interest rate of 1.71% at December 31, 2012 [Member], USD $)	1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2013 advance
Term Credit Agreement borrowings due 2019 at a variable interest rate of 1.71% at December 31, 2012 [Member]
Debt Instrument [Line Items]
Issuance date of debt instrument	December 2012
Maximum borrowing capacity	$ 640,000,000
Maturity date description	December 2019
Date through which borrowings can be made description	December 2017
Maximum number of advances		5
Basis spread on variable rate		1.50%
Commitment fee percentage		0.15%
Effective basis spread on variable rate		0.95%
Unused borrowing capacity		$ 140,000,000