COMMON SHARES (Tables)

12 Months Ended

Dec. 31, 2017

Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]

Schedule of Common Shares

	Number of Shares	Amount
	(thousands)	(millions of Canadian $)
Outstanding at January 1, 2015	779,479	16,320
Issuance of common shares for cash	—	—
Outstanding at December 31, 2015	779,479	16,320
Issuance of common shares for cash1	79,656	4,661
Outstanding at December 31, 2016	859,135	20,981
Issuance of common shares for cash	12,499	780
Outstanding at December 31, 2017	871,634	21,761

1 Proceeds of $2.5 billion were used to finance the acquisition of Columbia and proceeds of $2.0 billion were used to repay a portion of the US$6.9 billion acquisition bridge facilities.

Schedule of Options Valuation Assumptions

TransCanada used a binomial model for determining the fair value of options granted applying the following weighted average assumptions:

year ended December 31	2017		2016		2015
Weighted average fair value	$7.22		$5.67		$6.45
Expected life (years)	5.7		5.8		5.8
Interest rate	1.2	%	0.7	%	1.1	%
Volatility1	18	%	21	%	18	%
Dividend yield	3.6	%	4.9	%	3.7	%
Forfeiture rate2	—		5	%	5	%

1 Volatility is derived based on the average of both the historical and implied volatility of the Company's common shares.

2 On January 1, 2017, TransCanada made an election to account for forfeitures when they occur as a result of new GAAP guidance. Refer to Note 3, Accounting changes, for further information.

Schedule of Additional Option Information

period.

The following table summarizes additional stock option inf