Borrowings - Additional Information (Detail) - USD ($) $ in Thousands	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
Repurchase Agreement Counterparty
Collateral ratio									109.00%	111.00%
Net loss on extinguishment of debt	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 26	$ 0	$ 26	$ 6,814
Senior Exchangeable Note
Repurchase Agreement Counterparty
Debt retirement amount										143,400	256,600
Repurchase price					$ 143,400					143,400	262,100
Net loss on extinguishment of debt										$ 26	$ 6,800
Federal Home Loan Bank of Indianapolis
Repurchase Agreement Counterparty
Outstanding secured advances from FHLBI	$ 1,650,000								1,650,000
Average outstanding borrowings from FHLBI									$ 1,650,000
FHLBI weighted average interest rate on advances									2.52%
FHLBI weighted average maturity of advances									4 years 3 months 18 days