Investments in Marketable Debt Securities (Tables)

12 Months Ended

Dec. 31, 2017

Investments Debt And Equity Securities [Abstract]

Marketable Debt Securities

Marketable debt securities consisted of the following at December 31:

	AMORTIZED		UNREALIZED		UNREALIZED		FAIR
2017	COST		GAINS		LOSSES		VALUE
U.S. tax-exempt securities	$	537.9			$	2.4	$	535.5
U.S. corporate securities		89.7	$	.2		.2		89.7
U.S. government and agency securities		48.9				.2		48.7
Non-U.S. corporate securities		459.4		1.3		1.4		459.3
Non-U.S. government securities		91.5		.3		.1		91.7
Other debt securities		142.8		.1		.7		142.2
	$	1,370.2	$	1.9	$	5.0	$	1,367.1

	AMORTIZED		UNREALIZED		UNREALIZED		FAIR
2016	COST		GAINS		LOSSES		VALUE
U.S. tax-exempt securities	$	597.9	$	.2	$	3.1	$	595.0
U.S. corporate securities		47.6		.2				47.8
U.S. government and agency securities		16.0						16.0
Non-U.S. corporate securities		306.9		1.5		.4		308.0
Non-U.S. government securities		97.6		.6				98.2
Other debt securities		75.9		.2		.2		75.9
	$	1,141.9	$	2.7	$	3.7	$	1,140.9

Marketable Debt Securities Continuous Unrealized Losses

Marketable debt securities with continuous unrealized losses and their related fair values were as follows:

At December 31,	2017				2016
	LESS THAN TWELVE MONTHS		TWELVE MONTHS OR GREATER		LESS THAN TWELVE MONTHS		TWELVE MONTHS OR GREATER
Fair value	$	908.5	$	18.4	$	615.5
Unrealized losses		4.8		.2		3.7

Contractual Maturities of Debt Securities

Contractual maturities on marketable debt securities at December 31, 2017were as follows:

	AMORTIZED		FAIR
Maturities:	COST		VALUE
Within one year	$	413.7	$	413.3
One to five years		939.6		936.9
More than ten years		16.9		16.9
	$	1,370.2	$	1,367.1