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Related Party Transactions and Major Customers (Details) - USD ($)
 3 Months Ended 12 Months Ended
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Related Party Transaction [Line Items]                      
Gross premiums written $ 905,479,000 $ 861,068,000 $ 1,476,908,000 $ 1,564,295,000 $ 547,755,000 $ 625,677,000 $ 977,343,000 $ 1,159,652,000 $ 4,807,750,000 $ 3,310,427,000 $ 2,797,540,000
Gross premiums earned                 4,348,261,000 2,779,796,000 2,307,219,000
Acquisition expenses 208,618,000 $ 202,181,000 $ 227,482,000 $ 123,951,000 120,465,000 $ 109,761,000 $ 105,052,000 $ 97,711,000 762,232,000 432,989,000 346,892,000
Net reinsurance costs and recoveries                 (1,124,757,000) (1,458,518,000) (1,558,960,000)
Total gross reserve for claims and claim expenses 9,384,349,000       6,076,271,000       9,384,349,000 6,076,271,000 5,080,408,000
Ceded premiums written                 1,426,257,000 1,178,525,000 926,215,000
Ceded premiums earned                 1,414,383,000 1,095,886,000 833,929,000
Reinsurance recoverable 2,791,297,000               2,791,297,000    
Equity method investment, dividends and distributions                 $ (36,500,000) $ (26,100,000) $ 29,700,000
Gross premiums written | Three Brokers                      
Related Party Transaction [Line Items]                      
Percent of gross premiums received                 79.60% 75.20% 76.40%
Gross premiums written | AON                      
Related Party Transaction [Line Items]                      
Percent of gross premiums received                 41.70% 40.70% 42.80%
Gross premiums written | Marsh                      
Related Party Transaction [Line Items]                      
Percent of gross premiums received                 27.10% 24.60% 23.80%
Gross premiums written | Willis Towers Watson                      
Related Party Transaction [Line Items]                      
Percent of gross premiums received                 10.80% 9.90% 9.80%
Tower Hill Companies                      
Related Party Transaction [Line Items]                      
Gross premiums written                 $ 39,800,000 $ 45,500,000 $ 39,100,000
Gross premiums earned                 40,700,000 43,800,000 35,700,000
Acquisition expenses                 6,100,000 7,100,000 5,100,000
Related party receivables 14,800,000       19,300,000       14,800,000 19,300,000  
Net reinsurance costs and recoveries                 37,700,000 111,200,000 94,400,000
Total gross reserve for claims and claim expenses 71,800,000       $ 98,800,000       71,800,000 98,800,000  
Ceded premiums written                 500,000    
Ceded premiums earned                 400,000    
Ceded premiums payable 0               0    
Policyholder benefits and claims incurred, ceded                 0    
Policyholder benefits and claims recovered, ceded                 41,800,000    
Reinsurance recoverable $ 21,800,000               21,800,000    
Equity method investment, dividends and distributions                 13,400,000 12,100,000 8,300,000
Top Layer Reinsurance Ltd.                      
Related Party Transaction [Line Items]                      
Equity method investment, dividends and distributions                 20,000,000.0 12,500,000 20,000,000.0
Top Layer Reinsurance Ltd. | Management Fee                      
Related Party Transaction [Line Items]                      
Revenue recorded                 $ 2,300,000 $ 2,700,000 $ 2,700,000