The Company's investments in marketable debt securities are classified as available-for-sale. These investments are stated at fair value and may include an allowance for credit losses. Changes in the allowance for credit losses are recognized in the current period and any unrealized gains or losses, net of tax, are included as a component of accumulated other comprehensive income (loss) (AOCI).

The Company utilizes third-party pricing services for all of its marketable debt security valuations. The Company reviews the pricing methodology used by the third‑party pricing services, including the manner employed to collect market information. On a quarterly basis, the Company also performs review and validation procedures on the pricing information received from the third‑party providers. These procedures help ensure the fair value information used by the Company is determined in accordance with applicable accounting guidance.

The Company evaluates its investment in marketable debt securities at the end of each reporting period to determine if a decline in fair value is the result of credit losses or unrealized losses. In assessing credit losses, the Company considers the collectability of principal and interest payments by monitoring changes to issuers’ credit ratings, specific credit events associated with individual issuers as well as the credit ratings of any financial guarantor. The Company considers its intent for selling the security and whether it is more likely than not the Company will be able to hold the security until the recovery of any credit losses and unrealized losses. Charges against the allowance for credit losses occurs when a security with credit losses is sold or the Company no longer intends to hold that security.

Marketable debt securities at March 31, 2020 and December 31, 2019 consisted of the following:

	AMORTIZED		UNREALIZED		UNREALIZED		FAIR
At March 31, 2020	COST		GAINS		LOSSES		VALUE
U.S. tax-exempt securities	$	306.1	$	1.6	$	.8	$	306.9
U.S. corporate securities		157.9		1.3		.6		158.6
U.S. government and agency securities		119.4		4.0				123.4
Non-U.S. corporate securities		299.8		1.0		2.6		298.2
Non-U.S. government securities		70.8		.4				71.2
Other debt securities		137.0		1.8		.1		138.7
	$	1,091.0	$	10.1	$	4.1	$	1,097.0

	AMORTIZED		UNREALIZED		UNREALIZED		FAIR
At December 31, 2019	COST		GAINS		LOSSES		VALUE
U.S. tax-exempt securities	$	318.1	$	2.2	$	.1	$	320.2
U.S. corporate securities		163.8		1.9				165.7
U.S. government and agency securities		128.4		.9				129.3
Non-U.S. corporate securities		347.7		2.3		.2		349.8
Non-U.S. government securities		72.3		.2		.1		72.4
Other debt securities		123.7		1.1		.1		124.7
	$	1,154.0	$	8.6	$	.5	$	1,162.1

The cost of marketable debt securities is adjusted for amortization of premiums and accretion of discounts to maturity. Amortization, accretion, interest and dividend income and realized gains and losses are included in investment income. The cost of securities sold is

based on the specific identification method. Gross realized gains were $.5 and $.2 and gross realized losses were $.2 and $.1 for the three months periods ended March 31, 2020 and 2019, respectively.

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

	March 31, 2020			December 31, 2019
	LESS THAN		TWELVE MONTHS	LESS THAN		TWELVE MONTHS
	TWELVE MONTHS		OR GREATER	TWELVE MONTHS		OR GREATER
Fair value	$	366.0		$	177.0	$	31.4
Unrealized losses		4.1			.4		.1

The unrealized losses on the investments above were due to higher market yields and decreased market liquidity. The Company did not identify any indicators of a credit loss in its assessments. Accordingly, no allowance for credit losses was recorded at March 31, 2020 and December 31, 2019. The Company does not currently intend, and it is more likely than not, that it will not be required to sell the investment securities before recovery of the unrealized losses. The Company expects that the contractual principal and interest will be received on the investment securities.

Contractual maturities of marketable debt securities at March 31, 2020 were as follows:

	AMORTIZED		FAIR
Maturities:	COST		VALUE
Within one year	$	217.5	$	218.0
One to five years		834.5		840.0
Six to ten years		23.0		23.0
More than ten years		16.0		16.0
	$	1,091.0	$	1,097.0

Marketable debt securities included $32.8 and $49.7 of variable rate demand obligations (VRDOs) at March 31, 2020 and December 31, 2019, respectively. VRDOs are debt instruments with long-term scheduled maturities which have interest rates that reset periodically. Actual maturities of VRDOs may differ from contractual maturities because these securities may be sold when interest rates are reset.