Deferred Compensation Plan (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended					3 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012	Mar. 31, 2016	Aug. 01, 2013	May 09, 2014	Mar. 31, 2014	Feb. 21, 2014	Jan. 22, 2014	Oct. 18, 2013	Mar. 08, 2013	Mar. 10, 2010
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
deferred compensation arrangement with individual, units authorized for issuance	28,500
Deferred Compensation Arrangement with Individual, Units Outstanding	28,500
Redemption premium added to debt face amount, RDU plan	$ 0.1		$ 1.4
RDU plan cash retention pool							15.0
Charge for payment of RDU Plan cash retention pool	0	[1]	7.5	[1]
Deferred Compensation Arrangement with Individual, Requisite Service Period	3 years 0 months 0 days
RDU Plan, Compensation Expense	8.8		16.8		8.4
RDU Plan, Liability	30.4		21.8
Subordinated Debt [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Debt Instrument, Repurchased Face Amount							324.0	42.5	50.0	20.0	30.0	155.0	50.0	28.5
2015 [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
RDU Plan, unrecognized compensation expense	5
2016 through 2017 [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
RDU Plan, unrecognized compensation expense	3
Scenario, Forecast [Member]
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]
Second RDU Cash Retention Payment						$ 7.5

[1] See Note 12 for additional discussion of this transaction.