Fair Value Measurements and the Fair Value Option - Derivatives Financial Instruments (Details) $ in Millions	9 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2015 GBP (£)	Sep. 30, 2015 EUR (€)	Sep. 30, 2015 JPY (¥)
Derivative [Line Items]
Gain (loss) on fair value hedges recognized in earnings	$ 5.7
Gain (Loss) on Derivative Instruments [Member]
Derivative [Line Items]
Gain (loss) on derivative contracts that were unwound	3.2
Other Comprehensive Income (Loss) [Member]
Derivative [Line Items]
Unrealized gains recognized through OCI on derivative contracts settled during the period	10.6
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Fair Value	17.1
Designated as Hedging Instrument [Member] | Other Comprehensive Income (Loss) [Member]
Derivative [Line Items]
Change in Unrealized Gains (Losses)	3.2
Japanese Multifamily Portfolio and Resolutions of European Loan Pool Investments [Member] | Gain (Loss) on Derivative Instruments [Member]
Derivative [Line Items]
Gain (loss) on derivative contracts that were unwound	$ 15.1
Euro Member Countries, Euro [Member] | Foreign Exchange Forward, Trade Date 6/25/2014 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount | €			€ 20,000,000,000,000
Forward Rate/Strike Price	1.4471	1.4471	1.4471	1.4471
Fair Value	$ 5.2
Change in Unrealized Gains (Losses)	2.5
Euro Member Countries, Euro [Member] | Foreign Exchange Option, Trade Date 3/10/2015-3/19/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount | €			€ 130,000,000,000,000
Fair Value	(2.1)
Change in Unrealized Gains (Losses)	$ (2.1)
Euro Member Countries, Euro [Member] | Minimum [Member] | Foreign Exchange Option, Trade Date 3/10/2015-3/19/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Forward Rate/Strike Price	1.0700	1.0700	1.0700	1.0700
Euro Member Countries, Euro [Member] | Maximum [Member] | Foreign Exchange Option, Trade Date 3/10/2015-3/19/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Forward Rate/Strike Price	1.0960	1.0960	1.0960	1.0960
United Kingdom, Pounds [Member] | Foreign Exchange Forward, Trade Date 2/25/2014-8/10/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount | £		£ 103,000,000,000,000
Fair Value	$ 10.5
Change in Unrealized Gains (Losses)	6.7
United Kingdom, Pounds [Member] | Foreign Exchange Option, Trade Date 1/7/2015-8/17/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount | £		£ 100,200,000,000,000
Fair Value	1.9
Change in Unrealized Gains (Losses)	$ 0.7
United Kingdom, Pounds [Member] | Minimum [Member] | Foreign Exchange Forward, Trade Date 2/25/2014-8/10/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Forward Rate/Strike Price	1.5578	1.5578	1.5578	1.5578
United Kingdom, Pounds [Member] | Minimum [Member] | Foreign Exchange Option, Trade Date 1/7/2015-8/17/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Forward Rate/Strike Price	1.4235	1.4235	1.4235	1.4235
United Kingdom, Pounds [Member] | Maximum [Member] | Foreign Exchange Forward, Trade Date 2/25/2014-8/10/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Forward Rate/Strike Price	1.6371	1.6371	1.6371	1.6371
United Kingdom, Pounds [Member] | Maximum [Member] | Foreign Exchange Option, Trade Date 1/7/2015-8/17/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Forward Rate/Strike Price	1.5434	1.5434	1.5434	1.5434
Japan, Yen [Member] | Foreign Exchange Forward, Trade Date 6/23/2015-8/21/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount | ¥				¥ 649,000,000,000,000
Fair Value	$ (0.1)
Change in Unrealized Gains (Losses)	$ (0.1)
Japan, Yen [Member] | Minimum [Member] | Foreign Exchange Forward, Trade Date 6/23/2015-8/21/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Forward Rate/Strike Price	111.26	111.26	111.26	111.26
Japan, Yen [Member] | Maximum [Member] | Foreign Exchange Forward, Trade Date 6/23/2015-8/21/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Forward Rate/Strike Price	121.51	121.51	121.51	121.51
KWE [Member] | Euro Member Countries, Euro [Member] | Foreign Exchange Forward, Trade Date 6/18/2014-6/29/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount | €			€ 347,477,119,000,000
Fair Value	$ 4.4
Change in Unrealized Gains (Losses)	(1.8)
KWE [Member] | Euro Member Countries, Euro [Member] | Foreign Exchange Option, Trade Date 3/13/2015-6/3/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount | €			€ 175,000,000,000,000
Fair Value	(2.7)
Change in Unrealized Gains (Losses)	$ (2.7)
KWE [Member] | Euro Member Countries, Euro [Member] | Minimum [Member] | Foreign Exchange Forward, Trade Date 6/18/2014-6/29/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Forward Rate/Strike Price	0.7110	0.7110	0.7110	0.7110
KWE [Member] | Euro Member Countries, Euro [Member] | Minimum [Member] | Foreign Exchange Option, Trade Date 3/13/2015-6/3/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Forward Rate/Strike Price	0.7070	0.7070	0.7070	0.7070
KWE [Member] | Euro Member Countries, Euro [Member] | Maximum [Member] | Foreign Exchange Forward, Trade Date 6/18/2014-6/29/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Forward Rate/Strike Price	0.8621	0.8621	0.8621	0.8621
KWE [Member] | Euro Member Countries, Euro [Member] | Maximum [Member] | Foreign Exchange Option, Trade Date 3/13/2015-6/3/2015 [Member] | Designated as Hedging Instrument [Member]
Derivative [Line Items]
Forward Rate/Strike Price	0.7500	0.7500	0.7500	0.7500