Debt and Credit Facilities (Schedule of Credit Facilities) (Details) £ in Thousands, $ in Thousands	Dec. 31, 2016 USD ($)	Dec. 31, 2016 GBP (£)	Mar. 31, 2016 USD ($)	Mar. 31, 2016 GBP (£)
Line of Credit Facility [Line Items]
Credit facilities outstanding			$ 770,593	£ 100,000
Wells Fargo | Standby Letter of Credit Facility
Line of Credit Facility [Line Items]
Letters of credit outstanding	$ 140,829
National Australia Bank Limited | Standby Letter of Credit Facility
Line of Credit Facility [Line Items]
Letters of credit outstanding	4,855
Citibank Europe PLC | Bilateral Facility | Letter of Credit | Bilateral Facility Participants
Line of Credit Facility [Line Items]
Letters of credit outstanding	244,909
Citibank Europe PLC | Specialty Risks FAL Facility | Letter of Credit | Syndicate 1458
Line of Credit Facility [Line Items]
Letters of credit outstanding | £		£ 10,000
Bank Of Montreal, Citibank Europe and ING Bank | Renaissance Reinsurance FAL Facility | Letter of Credit | Syndicate 1458
Line of Credit Facility [Line Items]
Letters of credit outstanding	380,000	£ 90,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility, outstanding amount	$ 0