Long-term Investments (Tables)

12 Months Ended

Dec. 31, 2017

Financial Instruments [Abstract]

Disclosure of long-term investments

LONG-TERM INVESTMENTS

($ millions)	2017		2016
Investments in public companies, beginning of year	28.3		22.8
Acquired through capital dispositions	40.2		—
Unrealized gain (loss) recognized in other income (loss)	(3.4	)	5.5
Investments in public companies, end of year	65.1		28.3
Investment in private company, beginning of year	7.5		7.5
Unrealized gain (loss) recognized in other income (loss)	—		—
Investment in private company, end of year (1)	7.5		7.5
Long-term investments, end of year	72.6		35.8

(1) The investment in a private company was previously valued based primarily on recent trading activity in the company’s common shares, which resulted in a Level 2 fair value. At December 31, 2017, the investment was valued based on an estimate of the net asset value of the company’s common shares. Therefore, the fair value was reclassified to Level 3.