Related Party Transactions (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Related Party Transaction [Line Items]
Expenses from transactions with related party	$ (2.5)	$ (3.4)	$ (2.6)
Maximum
Related Party Transaction [Line Items]
Expenses from transactions with related party	$ (1.9)	$ (2.0)	$ (1.9)
WLR Funds
Related Party Transaction [Line Items]
Percentage of ownership by related party	8.20%	10.20%	10.90%
Wellington Management Company LLP
Related Party Transaction [Line Items]
Percentage of ownership by related party	6.60%	8.60%	9.60%