Schedule of Aggregate Amount of Senior Unsecured Notes Outstanding (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Dec. 31, 2013 Y	Mar. 31, 2013 Y
September 2013 Senior Unsecured Notes
Debt [Line Items]
Aggregate debt outstanding	$ 500,000
Weighted average remaining life in years	9.6
Weighted average coupon rate on notes outstanding	4.86%
Fair value of debt outstanding	495,276
August 2011 Senior Unsecured Notes
Debt [Line Items]
Aggregate debt outstanding	165,000	165,000
Weighted average remaining life in years	6.8	7.6
Weighted average coupon rate on notes outstanding	4.42%	4.42%
Fair value of debt outstanding	165,997	179,802
September 2010 Senior Unsecured Notes
Debt [Line Items]
Aggregate debt outstanding	425,000	425,000
Weighted average remaining life in years	5.9	6.6
Weighted average coupon rate on notes outstanding	4.25%	4.25%
Fair value of debt outstanding	428,873	458,520
July 2003 Senior Unsecured Notes
Debt [Line Items]
Aggregate debt outstanding	35,000	175,000
Weighted average remaining life in years	1.6	0.7
Weighted average coupon rate on notes outstanding	4.61%	4.47%
Fair value of debt outstanding	$ 36,263	$ 178,227