Employee and Non-Employee Director Stock and Benefit Plans - Share Awards (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Additional disclosures
Tax shortfalls	$ 0.9	$ 0.2
Restricted Stock Unit Awards
Total Number of Shares
Outstanding at the beginning of the period (in shares)	2,659,228	2,438,227	3,061,335
Granted (in shares)	2,196,191	1,477,389	1,023,811
Vested (in shares)	(1,159,047)	(1,123,720)	(1,203,142)
Forfeited (in shares)	(525,336)	(132,668)	(443,777)
Outstanding at the end of the period (in shares)	3,171,036	2,659,228	2,438,227
Weighted Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share)	$ 13.51	$ 12.31	$ 13.25
Granted (in dollars per share)	18.79	15.45	12.00
Vested (in dollars per share)	13.49	13.63	14.47
Forfeited (in dollars per share)	16.10	12.10	12.20
Outstanding at the end of the period (in dollars per share)	$ 16.75	$ 13.51	$ 12.31
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture	1,159,047	1,123,720	1,203,142
Unrecognized compensation costs	$ 34.2
Weighted average period over which unrecognized compensation costs are expected to be recognized	2 years 2 months 12 days
Grant date fair value	$ 24.9
Tax benefits related to the vesting	2.9	$ 1.3	$ 0.0
Tax shortfalls	0.3	0.4	1.2
Share-based compensation expense	16.4	15.0	13.0
Tax benefit related to share-based compensation expense	$ 6.6	$ 6.0	$ 5.2
Performance-Based Restricted Stock Units
Total Number of Shares
Outstanding at the beginning of the period (in shares)	438,022	458,743	677,831
Granted (in shares)	263,766	219,644	91,932
Vested (in shares)	(97,112)	(145,786)	(64,865)
Forfeited (in shares)	(227,201)	(94,579)	(246,155)
Outstanding at the end of the period (in shares)	377,475	438,022	458,743
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture	97,112	145,786	64,865
Market-Based Restricted Stock Units
Total Number of Shares
Outstanding at the end of the period (in shares)	196,862
Time-Based Restricted Stock Units
Total Number of Shares
Outstanding at the beginning of the period (in shares)	2,221,206	1,979,484	2,383,504
Granted (in shares)	1,932,425	1,257,745	931,879
Vested (in shares)	(1,061,935)	(977,934)	(1,138,277)
Forfeited (in shares)	(298,135)	(38,089)	(197,622)
Outstanding at the end of the period (in shares)	2,793,561	2,221,206	1,979,484
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture	1,061,935	977,934	1,138,277
ROE-Based Restricted Stock Units
Total Number of Shares
Outstanding at the end of the period (in shares)	180,613
Phantom share awards
Total Number of Shares
Outstanding at the beginning of the period (in shares)	358,349	671,762	695,710
Granted (in shares)			249,365
Vested (in shares)	(257,279)	(310,121)	(245,296)
Forfeited (in shares)	(43,272)	(3,292)	(28,017)
Outstanding at the end of the period (in shares)	57,798	358,349	671,762
Weighted Average Grant Date Fair Value
Outstanding at the beginning of the period (in dollars per share)	$ 11.61	$ 12.74	$ 13.72
Granted (in dollars per share)			12.01
Vested (in dollars per share)	11.58	14.06	14.52
Forfeited (in dollars per share)	10.99	11.73	14.92
Outstanding at the end of the period (in dollars per share)	$ 12.24	$ 11.61	$ 12.74
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture	257,279	310,121	245,296
Unrecognized compensation costs	$ 0.1
Weighted average period over which unrecognized compensation costs are expected to be recognized	1 month 24 days
2007 Plan | Phantom share awards
Additional disclosures
Share-based compensation expense	$ 2.2	$ 3.5	$ 7.3
Tax benefit related to share-based compensation expense	$ 0.8	$ 0.9	$ 2.0
Sale of subsidiaries | Energy research business | Restricted Stock Unit Awards
Total Number of Shares
Vested (in shares)	(113,718)
Additional disclosures
Number of unvested shares converted into vested shares upon divestiture	113,718
Other revenue | Sale of subsidiaries | Energy research business
Additional disclosures
Cost to modify the vesting schedule	$ 1.4