Consolidated Investment Entities - Consolidation of Investment Entities into the Consolidated Balance Sheets (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Total investments and cash	$ 93,364.7	$ 89,891.6
Other assets	16,255.7	15,305.1
Assets held in consolidated investment entities	11,323.2	8,999.4
Assets held in separate accounts	106,007.8	106,827.1
Total assets	226,951.4	221,023.2
Future policy benefits and contract owner account balances	85,010.7	84,006.7
Other liabilities	9,213.8	8,610.5
Liabilities held in consolidated investment entities	8,195.9	6,064.9
Liabilities related to separate accounts	106,007.8	106,827.1
Total liabilities	208,428.2	205,509.2
Equity attributable to common shareholders	16,087.5	13,253.8
Appropriated-consolidated investment entities	20.4	18.4
Noncontrolling interest	2,415.3	2,241.8
Total liabilities and shareholder's equity	226,951.4	221,023.2
Before Consolidation
Condensed Financial Statements, Captions [Line Items]
Total investments and cash	94,059.1	90,569.7
Other assets	16,255.7	15,305.1
Assets held in consolidated investment entities	0	0
Assets held in separate accounts	106,007.8	106,827.1
Total assets	216,322.6	212,701.9
Future policy benefits and contract owner account balances	85,010.7	84,006.7
Other liabilities	9,216.5	8,614.3
Liabilities held in consolidated investment entities	0.1	0
Liabilities related to separate accounts	106,007.8	106,827.1
Total liabilities	200,235.1	199,448.1
Equity attributable to common shareholders	16,087.5	13,253.8
Appropriated-consolidated investment entities	0	0
Noncontrolling interest	0	0
Total liabilities and shareholder's equity	216,322.6	212,701.9
CLOs
Condensed Financial Statements, Captions [Line Items]
Total investments and cash	0	0
Other assets	0	0
Assets held in consolidated investment entities	7,466.2	5,690.9
Assets held in separate accounts	0	0
Total assets	7,466.2	5,690.9
Future policy benefits and contract owner account balances	0	0
Other liabilities	0	0
Liabilities held in consolidated investment entities	7,445.8	5,672.5
Liabilities related to separate accounts	0	0
Total liabilities	7,445.8	5,672.5
Equity attributable to common shareholders	0	0
Appropriated-consolidated investment entities	20.4	18.4
Noncontrolling interest	0	0
Total liabilities and shareholder's equity	7,466.2	5,690.9
VOEs
Condensed Financial Statements, Captions [Line Items]
Total investments and cash	0	0
Other assets	0	0
Assets held in consolidated investment entities	3,859.7	3,288.1
Assets held in separate accounts	0	0
Total assets	3,859.7	3,288.1
Future policy benefits and contract owner account balances	0	0
Other liabilities	0	0
Liabilities held in consolidated investment entities	796.6	435.4
Liabilities related to separate accounts	0	0
Total liabilities	796.6	435.4
Equity attributable to common shareholders	3,063.1	2,852.7
Appropriated-consolidated investment entities	0	0
Noncontrolling interest	0	0
Total liabilities and shareholder's equity	3,859.7	3,288.1
CLOs Adjustments
Condensed Financial Statements, Captions [Line Items]
Total investments and cash	(46.6)	(43.0)
Other assets	0	0
Assets held in consolidated investment entities	0	0
Assets held in separate accounts	0	0
Total assets	(46.6)	(43.0)
Future policy benefits and contract owner account balances	0	0
Other liabilities	0	0
Liabilities held in consolidated investment entities	(46.6)	(43.0)
Liabilities related to separate accounts	0	0
Total liabilities	(46.6)	(43.0)
Equity attributable to common shareholders	0	0
Appropriated-consolidated investment entities	0	0
Noncontrolling interest	0	0
Total liabilities and shareholder's equity	(46.6)	(43.0)
VOEs Adjustments
Condensed Financial Statements, Captions [Line Items]
Total investments and cash	(647.8)	(635.1)
Other assets	0	0
Assets held in consolidated investment entities	(2.7)	20.4
Assets held in separate accounts	0	0
Total assets	(650.5)	(614.7)
Future policy benefits and contract owner account balances	0	0
Other liabilities	(2.7)	(3.8)
Liabilities held in consolidated investment entities	0	0
Liabilities related to separate accounts	0	0
Total liabilities	(2.7)	(3.8)
Equity attributable to common shareholders	(3,063.1)	(2,852.7)
Appropriated-consolidated investment entities	0	0
Noncontrolling interest	2,415.3	2,241.8
Total liabilities and shareholder's equity	$ (650.5)	$ (614.7)