Investments - Summary of Unaudited Financial Information and Unaudited Financial Position of Limited Partnerships (Detail) - USD ($) $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating results:
Total income	$ 52,997	$ 61,743	$ 58,813	$ 57,739	$ 61,623	$ 47,490	$ 67,580	$ 67,713	$ 72,371	$ 69,808	$ 61,520	$ 60,747	$ 231,292	$ 244,406	$ 264,446
Limited Partnership [Member]
Operating results:
Total income													1,821,935	409,169	310,998
Total expenses													146,079	105,281	185,126
Net income (loss)													1,675,856	303,888	$ 125,872
Balance Sheet:
Total assets	6,689,792				4,381,321								6,689,792	4,381,321
Total liabilities	$ 394,029				$ 382,310								$ 394,029	$ 382,310