Expected maturities may differ from contractual maturities as borrowers may have the right to call or prepay obligations with or without call or prepayment penalties.

We deposit bonds with insurance regulatory authorities to meet statutory requirements. The adjusted cost of bonds on deposit with insurance regulatory authorities was $28.2 million and $16.8 million at September 30, 2017 and March 31, 2017, respectively.

Available-for-Sale Investments

Available-for-sale investments at September 30, 2017 were as follows:

		Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses More than 12 Months		Gross Unrealized Losses Less than 12 Months		Estimated Market Value
		(Unaudited)
		(In thousands)
U.S. treasury securities and government obligations	$	123,503	$	3,468	$	–	$	(1,176)	$	125,795
U.S. government agency mortgage-backed securities		26,812		1,355		(2)		(2)		28,163
Obligations of states and political subdivisions		174,665		10,626		(10)		(223)		185,058
Corporate securities		1,332,938		48,148		(3,786)		(3,631)		1,373,669
Mortgage-backed securities		89,858		1,958		–		–		91,816
Redeemable preferred stocks		13,789		539		–		(25)		14,303
Common stocks		15,732		9,242		(10)		–		24,964
	$	1,777,297	$	75,336	$	(3,808)	$	(5,057)	$	1,843,768

Available-for-sale investments at March 31, 2017 were as follows:

		Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses More than 12 Months		Gross Unrealized Losses Less than 12 Months		Estimated Market Value

		(In thousands)
U.S. treasury securities and government obligations	$	123,474	$	2,892	$	–	$	(1,675)	$	124,691
U.S. government agency mortgage-backed securities		27,908		1,070		(6)		(377)		28,595
Obligations of states and political subdivisions		159,417		9,466		(23)		(424)		168,436
Corporate securities		1,263,703		32,901		(5,731)		(13,837)		1,277,036
Mortgage-backed securities		26,577		515		–		(5)		27,087
Redeemable preferred stocks		13,789		168		–		(468)		13,489
Common stocks		15,732		8,728		(10)		(16)		24,434
	$	1,630,600	$	55,740	$	(5,770)	$	(16,802)	$	1,663,768

The available-for-sale tables include gross unrealized losses that are not deemed to be other-than-temporarily impaired, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position.

We sold available-for-sale securities with a fair value of $68.5 million during the first six months of fiscal 2018. The gross realized gains on these sales totaled $2.6 million. The gross realized losses on these sales totaled $0.1 million.

The unrealized losses of more than twelve months in the available-for-sale tables are considered temporary declines. We track each investment with an unrealized loss and evaluate it on an individual basis for other-than-temporary impairments including obtaining corroborating opinions from third party sources, performing trend analysis and reviewing management’s future plans. Certain of these investments may have declines determined by management to be other-than-temporary and we recognize these write-downs, if any, through earnings. There were no write downs in the second quarter or for the first six months of fiscal 2018 or 2017.

The investment portfolio primarily consists of corporate securities and obligations of states and political subdivisions. We believe we monitor our investments as appropriate. Our methodology of assessing other-than-temporary impairments is based on security-specific analysis as of the balance sheet date and considers various factors including the length of time to maturity, the extent to which the fair value has been less than the cost, the financial condition and the near-term prospects of the issuer, and whether the debtor is current on its contractually obligated interest and principal payments. Nothing has come to management’s attention that would lead to the belief that each issuer would not have the ability to meet the remaining contractual obligations of the security, including payment at maturity. We have the ability and intent not to sell our fixed maturity and common stock investments for a period of time sufficient to allow us to recover our costs.

The portion of other-than-temporary impairment related to a credit loss is recognized in earnings. The significant inputs utilized in the evaluation of mortgage backed securities credit losses include ratings, delinquency rates, and prepayment activity. The significant inputs utilized in the evaluation of asset backed securities credit losses include the time frame for principal recovery and the subordination and value of the underlying collateral.

There were no credit losses recognized in earnings for which a portion of an other-than-temporary impairment was recognized in accumulated other comprehensive income (loss) for the second quarter or first six months of fiscal 2018 and fiscal 2017, respectively.

The adjusted cost and estimated market value of available-for-sale investments by contractual maturity were as follows:

		September 30, 2017				March 31, 2017
		Amortized Cost		Estimated Market Value		Amortized Cost		Estimated Market Value
		(Unaudited)
		(In thousands)
Due in one year or less	$	40,057	$	40,428	$	35,399	$	35,795
Due after one year through five years		366,506		377,028		324,286		333,016
Due after five years through ten years		609,614		628,689		598,232		607,184
Due after ten years		641,741		666,540		616,585		622,763
		1,657,918		1,712,685		1,574,502		1,598,758

Mortgage backed securities		89,858		91,816		26,577		27,087
Redeemable preferred stocks		13,789		14,303		13,789		13,489
Common stocks		15,732		24,964		15,732		24,434
	$	1,777,297	$	1,843,768	$	1,630,600	$	1,663,768