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Investments (Tables)
 6 Months Ended
Sep. 30, 2015
Investments [Abstract]  
Summary of available-for-sale securities
The following is a summary of available-for-sale securities at September 30, 2015 (amounts in thousands):
 
Available-for-sale Securities
 
Adjusted
Cost
 
Gross
Unrealized
Gains
 
Gross
Unrealized
Losses
 
Estimated
Fair Value
Government agency bonds
$
866,941

 
$
1,304

 
$
(51
)
 
$
868,194

Municipal bonds
42,579

 
11

 
(516
)
 
42,074

Auction rate securities
9,825

 
—

 
—

 
9,825

Corporate bonds and debt
1,127,390

 
1,323

 
(1,430
)
 
1,127,283

 
$
2,046,735

 
$
2,638

 
$
(1,997
)
 
$
2,047,376

 
The following is a summary of available-for-sale securities at March 31, 2015 (amounts in thousands):
 
Available-for-sale Securities
 
Adjusted
Cost
 
Gross
Unrealized
Gains
 
Gross
Unrealized
Losses
 
Estimated
Fair Value
Government agency bonds
$
741,780

 
$
676

 
$
(200
)
 
$
742,256

Municipal bonds
41,552

 
155

 
(9
)
 
41,698

Auction rate securities
9,825

 
—

 
—

 
9,825

Time deposits (1)
506

 
—

 
—

 
506

Corporate bonds and debt
924,818

 
2,376

 
(265
)
 
926,929

Marketable equity securities
1,362

 
11,804

 
—

 
13,166

 
$
1,719,843

 
$
15,011

 
$
(474
)
 
$
1,734,380
Summary of available-for-sale securities in a continuous unrealized loss position
The following tables show all investments in an unrealized loss position for which an other-than-temporary impairment has not been recognized and the related gross unrealized losses and fair value, aggregated by investment category and the length of time that the individual securities have been in a continuous unrealized loss position (amounts in thousands):
 
September 30, 2015
 
Less than 12 Months
 
12 Months or Greater
 
Total
 
Fair Value
 
Unrealized Loss
 
Fair Value
 
Unrealized Loss
 
Fair Value
 
Unrealized Loss
Government agency bonds
$
140,775

 
$
(51
)
 
$
10,000

 
$
—

 
$
150,775

 
$
(51
)
Municipal bonds
34,222

 
(516
)
 
—

 
—

 
34,222

 
(516
)
Corporate bonds and debt
391,344

 
(1,361
)
 
26,828

 
(69
)
 
418,172

 
(1,430
)
 
$
566,341

 
$
(1,928
)
 
$
36,828

 
$
(69
)
 
$
603,169

 
$
(1,997
)


 
March 31, 2015
 
Less than 12 Months
 
12 Months or Greater
 
Total
 
Fair Value
 
Unrealized Loss
 
Fair Value
 
Unrealized Loss
 
Fair Value
 
Unrealized Loss
Government agency bonds
$
162,948

 
$
(142
)
 
$
29,942

 
$
(58
)
 
$
192,890

 
$
(200
)
Municipal bonds
13,318

 
(9
)
 
—

 
—

 
13,318

 
(9
)
Corporate bonds and debt
163,095

 
(219
)
 
19,021

 
(46
)
 
182,116

 
(265
)
 
$
339,361

 
$
(370
)
 
$
48,963

 
$
(104
)
 
$
388,324

 
$
(474
)
Summary of amortized cost and estimated fair value of available-for-sale securities, by maturity
The amortized cost and estimated fair value of the available-for-sale securities at September 30, 2015, by contractual maturity, excluding corporate debt of $6.2 million, which has no contractual maturity, are shown below (amounts in thousands).  Expected maturities can differ from contractual maturities because the issuers of the securities may have the right to prepay obligations without prepayment penalties, and the Company views its available-for-sale securities as available for current operations.
 
Adjusted
Cost
 
Gross
Unrealized
Gains
 
Gross
Unrealized
Losses
 
Estimated
Fair Value
Available-for-sale
 
 
 
 
 
 
 
Due in one year or less
$
377,682

 
$
650

 
$
(42
)
 
$
378,290

Due after one year and through five years
1,520,303

 
1,942

 
(1,954
)
 
1,520,291

Due after five years and through ten years
132,735

 
46

 
(1
)
 
132,780

Due after ten years
9,825

 
—

 
—

 
9,825

 
$
2,040,545

 
$
2,638

 
$
(1,997
)
 
$
2,041,186