Variable Interest Entities and Noncontrolling Interests - Other variable interest entity disclosures (Details) - USD ($) $ in Thousands		Mar. 31, 2021		Dec. 31, 2020
Variable Interest Entity [Line Items]
Total VIE Assets		$ 44,204,267		$ 43,282,297
Maximum Exposure to Loss, Total		2,452,414		2,232,609
Variable Interest Entity, Not Primary Beneficiary
Variable Interest Entity [Line Items]
Total VIE Assets		4,200,317		3,859,058
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability		(7,610)		(7,969)
Maximum Exposure to Loss - Off-Balance Sheet		63,659		56,320
Maximum Exposure to Loss, Total		56,049		48,351
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2017-1 Ltd. (Oct-17)
Variable Interest Entity [Line Items]
Total VIE Assets		145,573		145,573
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability		(306)		(245)
Maximum Exposure to Loss - Off-Balance Sheet		1,152		844
Maximum Exposure to Loss, Total		846		599
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2018-1 Ltd. (Apr-18)
Variable Interest Entity [Line Items]
Total VIE Assets		250,095		250,095
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability		(1,081)		(903)
Maximum Exposure to Loss - Off-Balance Sheet		2,885		2,245
Maximum Exposure to Loss, Total		1,804		1,342
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2018-2 Ltd. (Aug-18)
Variable Interest Entity [Line Items]
Total VIE Assets		66,747		108,395
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability		(32)		(138)
Maximum Exposure to Loss - Off-Balance Sheet		62		280
Maximum Exposure to Loss, Total		30		142
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2018-3 Ltd. (Oct-18)
Variable Interest Entity [Line Items]
Total VIE Assets		302,563		302,563
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability		(1,604)		(1,320)
Maximum Exposure to Loss - Off-Balance Sheet		4,277		3,262
Maximum Exposure to Loss, Total		2,673		1,942
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2019-1 Ltd. (Mar-19)
Variable Interest Entity [Line Items]
Total VIE Assets		219,256		219,256
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability		(1,117)		(1,361)
Maximum Exposure to Loss - Off-Balance Sheet		7,991		8,461
Maximum Exposure to Loss, Total		6,874		7,100
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2019-2 Ltd. (Apr-19)
Variable Interest Entity [Line Items]
Total VIE Assets		398,316		398,316
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability		(780)		(730)
Maximum Exposure to Loss - Off-Balance Sheet		6,577		5,201
Maximum Exposure to Loss, Total		5,797		4,471
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2019-3 Ltd. (Jul-19)
Variable Interest Entity [Line Items]
Total VIE Assets		528,084		528,084
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability		(898)		(861)
Maximum Exposure to Loss - Off-Balance Sheet		6,047		5,079
Maximum Exposure to Loss, Total		5,149		4,218
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2019-4 Ltd. (Oct-19)
Variable Interest Entity [Line Items]
Total VIE Assets		468,737		468,737
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability		(908)		(890)
Maximum Exposure to Loss - Off-Balance Sheet		8,034		6,676
Maximum Exposure to Loss, Total		7,126		5,786
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2020-1 Ltd. (Jun-20)
Variable Interest Entity [Line Items]
Total VIE Assets	[1]	132,881		275,068
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability	[1]	(58)		(178)
Maximum Exposure to Loss - Off-Balance Sheet	[1]	294		1,012
Maximum Exposure to Loss, Total	[1]	236		834
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2020-1 Ltd. (Jun-20) | Funded by directly provided capacity
Variable Interest Entity [Line Items]
Initial Coverage, Amount		79,000
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2020-2 Ltd (Sep-20)
Variable Interest Entity [Line Items]
Total VIE Assets	[2]	368,797		423,420
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability	[2]	(370)		(556)
Maximum Exposure to Loss - Off-Balance Sheet	[2]	6,077		6,839
Maximum Exposure to Loss, Total	[2]	5,707		6,283
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2020-2 Ltd (Sep-20) | Funded by directly provided capacity
Variable Interest Entity [Line Items]
Initial Coverage, Amount		26,000
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2020-3 Ltd. (Nov-20)
Variable Interest Entity [Line Items]
Total VIE Assets	[3]	418,158		418,158
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability	[3]	(433)		(631)
Maximum Exposure to Loss - Off-Balance Sheet	[3]	9,527		9,605
Maximum Exposure to Loss, Total	[3]	9,094		8,974
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2020-3 Ltd. (Nov-20) | Funded by directly provided capacity
Variable Interest Entity [Line Items]
Initial Coverage, Amount		34,000
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2020-4 Ltd. (Dec-20)
Variable Interest Entity [Line Items]
Total VIE Assets	[4]	321,393		321,393
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability	[4]	(23)		(156)
Maximum Exposure to Loss - Off-Balance Sheet	[4]	5,969		6,816
Maximum Exposure to Loss, Total	[4]	5,946		6,660
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2020-4 Ltd. (Dec-20) | Funded by directly provided capacity
Variable Interest Entity [Line Items]
Initial Coverage, Amount		16,000
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2021-1 Ltd. (Mar-21)
Variable Interest Entity [Line Items]
Total VIE Assets		579,717	[5]	0
Maximum Exposure to Loss - On-Balance Sheet (Asset) Liability		0	[5]	0
Maximum Exposure to Loss - Off-Balance Sheet		4,767	[5]	0
Maximum Exposure to Loss, Total		4,767	[5]	$ 0
Variable Interest Entity, Not Primary Beneficiary | Bellemeade 2021-1 Ltd. (Mar-21) | Funded by directly provided capacity
Variable Interest Entity [Line Items]
Initial Coverage, Amount		$ 64,000

[1]  An additional $79 million capacity was provided directly to Arch MI U.S. by a separate panel of reinsurers and is not reflected in this table

[2] An additional $26 million capacity was provided directly to Arch MI U.S. by a separate panel of reinsurers and is not reflected in this table.

[3] An additional $34 million capacity was provided directly to Arch MI U.S. by a separate panel of reinsurers and is not reflected in this table.

[4] An additional $16 million capacity was provided directly to Arch MI U.S. by a separate panel of reinsurers and is not reflected in this table.

[5] An additional $64 million capacity was provided directly to Arch MI U.S. by a separate panel of reinsurers and is not reflected in this table