Reinsurance - Summary of Total Assets and Maximum Exposure Loss (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Variable Interest Entity [Line Items]
Total VIE Assets	$ 5,202,724	$ 3,873,425
Variable Interest Entity
Variable Interest Entity [Line Items]
Total VIE Assets	1,679,188
Maximum exposure to loss, on - balance sheet	(4,345)
Maximum exposure to loss, off - balance sheet	346
Maximum exposure to loss, total	(3,999)
Radnor Re 2018-1 Ltd. | Variable Interest Entity
Variable Interest Entity [Line Items]
Total VIE Assets	242,123
Maximum exposure to loss, on - balance sheet	462
Maximum exposure to loss, off - balance sheet	32
Maximum exposure to loss, total	494
Radnor Re 2019-1 Ltd. | Variable Interest Entity
Variable Interest Entity [Line Items]
Total VIE Assets	325,537
Maximum exposure to loss, on - balance sheet	(2,071)
Maximum exposure to loss, off - balance sheet	56
Maximum exposure to loss, total	(2,015)
Radnor Re 2019-2 Ltd. | Variable Interest Entity
Variable Interest Entity [Line Items]
Total VIE Assets	216,480
Maximum exposure to loss, on - balance sheet	(1,509)
Maximum exposure to loss, off - balance sheet	26
Maximum exposure to loss, total	(1,483)
Radnor Re 2020-1 Ltd. | Variable Interest Entity
Variable Interest Entity [Line Items]
Total VIE Assets	495,889
Maximum exposure to loss, on - balance sheet	(1,046)
Maximum exposure to loss, off - balance sheet	123
Maximum exposure to loss, total	(923)
Radnor Re 2020-2 Ltd. | Variable Interest Entity
Variable Interest Entity [Line Items]
Total VIE Assets	399,159
Maximum exposure to loss, on - balance sheet	(181)
Maximum exposure to loss, off - balance sheet	109
Maximum exposure to loss, total	$ (72)