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Marketable securities (Tables)
 12 Months Ended
Mar. 31, 2013
Marketable securities [Abstract]  
Available-for-sale debt securities
Available-for-sale debt securities consist of the following:

 
March 31, 2013
 
(In thousands)
 
Estimated fair value
 
 
Gains in accumulated other comprehensive income
 
 
Losses in accumulated other comprehensive income
 
 
 
 
 
 
 
 
 
 
Current:
 
 
 
 
 
 
 
 
 
Municipal bonds and notes
 
$
34,025
 
 
$
34
 
 
 
 
Government agency bonds
 
 
87,227
 
 
 
125
 
 
$
( 10
)
Commercial paper
 
 
144,293
 
 
 
-
 
 
 
 
 
Certificates of deposit
 
 
47,977
 
 
 
-
 
 
 
( 2
)
Corporate bonds
 
 
425,676
 
 
 
1,286
 
 
 
( 33
)
Total current securities
 
 
739,198
 
 
 
1,445
 
 
 
( 45
)
 
 
 
 
 
 
 
 
 
 
 
 
Non-current:
 
 
 
 
 
 
 
 
 
 
 
 
Municipal bonds and notes
 
 
12,852
 
 
 
37
 
 
 
 
 
Government agency bonds
 
 
186,577
 
 
 
434
 
 
 
( 19
)
Certificates of deposit
 
 
22,999
 
 
 
-
 
 
 
 
 
Corporate bonds
 
 
1,084,194
 
 
 
5,290
 
 
 
(   2,150
)
Auction rate notes
 
 
3,198
 
 
 
 
 
 
 
( 752
)
Variable rate notes
 
 
1,500
 
 
 
 
 
 
 
( -
)
Total non-current securities
 
 
1,311,320
 
 
 
5,761
 
 
 
( 2,921
)
 
 
 
 
 
 
 
 
 
 
 
 
Total available-for-sale debt securities
 
$
2,050,518
 
 
$
7,206
 
 
$
( 2,966
)

 
March 31, 2012
 
(In thousands)
 
Estimated fair value
 
 
Gains in accumulated other comprehensive income
 
 
Losses in accumulated other comprehensive income
 
 
 
 
 
 
 
 
 
 
Current:
 
 
 
 
 
 
 
 
 
Municipal bonds and notes
 
$
33,723
 
 
$
52
 
 
 
 
Government agency bonds
 
 
92,829
 
 
 
123
 
 
 
 
Commercial paper
 
 
239,393
 
 
 
334
 
 
$
( 70
)
Certificates of deposit
 
 
91,819
 
 
 
320
 
 
 
 
 
Corporate bonds
 
 
210,852
 
 
 
76
 
 
 
( 79
)
Floating rate notes
 
 
178,939
 
 
 
281
 
 
 
( 22
)
Total current securities
 
 
847,555
 
 
 
1,186
 
 
 
( 171
)
 
 
 
 
 
 
 
 
 
 
 
 
Non-current:
 
 
 
 
 
 
 
 
 
 
 
 
Municipal bonds and notes
 
 
35,890
 
 
 
45
 
 
 
 
 
Government agency bonds
 
 
60,087
 
 
 
185
 
 
 
 
 
Commercial paper
 
 
14,682
 
 
 
111
 
 
 
 
 
Corporate bonds
 
 
305,697
 
 
 
779
 
 
 
( 82
)
Auction rate notes
 
 
25,089
 
 
 
 
 
 
 
 
 
Floating rate notes
 
 
254,193
 
 
 
 
 
 
 
( 10,547
)
Total non-current securities
 
 
695,638
 
 
 
1,120
 
 
 
( 10,629
)
 
 
 
 
 
 
 
 
 
 
 
 
Total available-for-sale debt securities
 
$
1,543,193
 
 
$
2,306
 
 
$
(10,800
)
 
Contractual maturities of available-for-sale debt securities
Contractual maturities of available-for-sale debt securities at March 31, 2013 are as follows:

(In thousands)
 
Estimated fair value
 
Within one year
 
$
739,198
 
1-5 years
 
 
1,303,416
 
5-10 years
 
 
-
 
After 10 years
 
 
7,904
 
 
$
2,050,518