Contract Owners' Account Balances by Mutual Fund Class and by Guaranteed Benefit Provisions (Detail) - Variable Annuity Contracts - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	$ 4,410,481	$ 4,333,817
Equity
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	2,717,985	2,589,651
Bond
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	842,104	877,990
Balanced
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	438,695	438,480
Money Market
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	411,697	427,696
GMDB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	2,239,510	2,177,733
GMDB | Equity
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	1,406,510	1,303,816
GMDB | Bond
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	388,218	418,368
GMDB | Balanced
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	342,216	341,637
GMDB | Money Market
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	102,566	113,912
GMDB and GMIB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	1,321,869	1,296,010
GMDB and GMIB | Equity
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	819,035	793,764
GMDB and GMIB | Bond
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	262,873	263,157
GMDB and GMIB | Balanced
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	91,878	91,138
GMDB and GMIB | Money Market
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	148,083	147,951
GMDB and GMWB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	481,412	493,866
GMDB and GMWB | Equity
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	279,455	284,597
GMDB and GMWB | Bond
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	112,727	118,392
GMDB and GMWB | Balanced
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	1,977	1,862
GMDB and GMWB | Money Market
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	87,253	89,015
GMWB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	201,325	201,415
GMWB | Equity
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	117,918	117,273
GMWB | Bond
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	47,596	47,229
GMWB | Balanced
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	1,455	1,165
GMWB | Money Market
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	34,356	35,748
GMIB
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	131,375	129,162
GMIB | Equity
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	76,830	73,648
GMIB | Bond
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	25,306	25,106
GMIB | Balanced
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	643	1,570
GMIB | Money Market
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	28,596	28,838
Non Guaranteed Obligations
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	34,990	35,631
Non Guaranteed Obligations | Equity
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	18,237	16,553
Non Guaranteed Obligations | Bond
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	5,384	5,738
Non Guaranteed Obligations | Balanced
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	526	1,108
Non Guaranteed Obligations | Money Market
Liabilities For Guarantees On Long Duration Contracts [Line Items]
Contract owners account balances, guaranteed benefit liabilities	$ 10,843	$ 12,232